The way the term 'consumerism' is used in your quote in the first bit does not seem to be the usual usage, so it feels a lot like equivocation to me. Consumerism is not consumption. Consumerism is not even just buying stuff that serves no purpose other than to make your life better. Consumerism is specifically buying frivolous stuff. Because of that, the first two paragraphs seems like useless window-dressing to me. No one is arguing that consumption is bad, I just ate lunch and it was delicious, now let's move on from that strawman.

With regards to frivolous consumption, there is a problem with regards to the definition of frivolous. I think the best way to think about this is to recognise that human wants and desires are quite malleable. Because of this, things that don't actually materially improve your life (eg. give you a good chance of living longer, free up significant portions of time, etc.) and instead are purchased primarily because buying the item gives a burst of pleasure, are fundamentally useless. Sure, having this item makes you happier, but so does just about any action that you can convince yourself is valuable. An example of such an item might be a fancy branded mechanical keyboard with just the right switches. There is no fundamental reason why such a keyboard would make me happier than, say, spending some quality time with my family, even though personally I do desire such items. The assumption in your quote is that frivolous purchases still provide conveniences, but I would argue many items really really don't! Buying a new iPhone every time your contract expires does not provide any new convenience over, say, a battery swap. You might be able to have fun playing with new games, or features, but I had way more fun playing PS2 games with my friends decades ago than I have on any modern phone game; it really doesn't matter. Neither do mechanical keyboards; if anything, the longer travel distance might worsen RSIs.

It is also important to recognise that due to the hedonic treadmill, you don't derive long-term enjoyment from buying things. After a while you get used to it; losing the item would bring you sadness, but the continued existence of the item no longer brings joy. Because of that, buying a durable item (eg. fancy keyboard) is actually far more similar to activities that bring transient enjoyment (hanging out with people) than one might imagine.

Now, if there are no negative externalities, none of this would matter. After all, the universe is cold and uncaring, why not have some fun, etc. However, there are. I mean, there's basically the whole climate thing going on, and the whole microplastics things, and producing more stuff has costs to society as a whole. However, even if we ignore that, if we zoom out a bit, there are costs. Society as a whole as some maximum level of productivity given by our total amount of technology, labour and human capital, land, and actual capital (eg. accumulated machinery, etc.). The more of this productivity is directed towards producing useless shit, the less we can direct towards actually making the world better, advancing technology, helping people, etc. Because of this, I strongly believe that if there is any consumption that provides utility that can be equivalently substituted by non-consumption, that consumption is a net negative for society. This is not to say I am a magical person of magical will-power. I buy shit that's useless. However, I recognise that I bought a thing that brings be less joy and wonder than a walk through the park after a spring shower, and maybe I should remind myself to do that more often.

Intruiging question - this reminds me of the very thought-provoking essay by Sarah Constanin defending individualism. I am not entirely sure what you mean by 'steelman consumerism' as it could refer to defending the concept's usefulness or defending a more object-level claim about the harmfulness of over-consumption or certain types of consumption (or it could mean something I have not thought of)? I'll try to address the first two briefly to the best of my ability.

As a descriptive concept, 'consumerism' serves a useful historical function in describing a change in the way people engaged in their consumptive habits around the 18th century. I think the core essence of this change is in two parts: (I) the scale of consumption and character of the things consumed; (1I) the relationship of identity to material consumer goods. The first relates to the undeniable fact that in some parts of the world, (eg, the developed west), a large proportion of the population has been able to engage in luxury spending. That is, buying things which do not contribute to basic living functions. Afaik, scholars do not suggest this is a change in human nature, but an observation that material plenty has enabled greater spending on luxury goods. In turn, this has led to consumption taking on a greater role in the economy and in the culture of society (eg, shopping mall becoming a centre of community planning). This leads to (II). Historians also suggest that around the 18th century to 'consume' took on a different role. Instead of being seen primarily in a negative light (eg, to extinguish, and as a part of the deadly sins), it took on a positive connotation of productivity and creativity. It became socially acceptable - and even advantageous - for large portions of the population to pursue consumption openly (conspicuous consumption) and with no other justification than fulfilling desires. In turn, this has led to a culture where consumption is encouraged in all parts of life; there are few moral limits on what you spend your money on; and people increasingly identify themselves, and their status, with their consumptive purchases. This is substantially different from the pre-18th century environment of highly moralised and restrictive consumption. Whether you think (I) and (II) are true - and potentially interesting - will shape your view on the usefulness of 'consumerism' as a general concept.  

In terms of the object-level criticism of 'consumerism', several (not necessarily compatible) arguments can be made. Note that many of these involve positing a kind of lexical ordering of goods/life-pursuits which you may disagree with, and hence will find unpersuasive. 

• Although 'consuming' is not itself bad, one should try as hard as possible to avoid relying, fixating, or focusing on material goods. This is to pursue 'higher-order' ends like spirituality and personal virtues which a focus on material consumption can distract from. Represented historically by stoics, buddhists, and many other varieties of religious ascetics. 
• A kind of secular version of the argument above suggesting that 'the most important things in life' (family, friendship, personal virtues, adventure, spontaneity, etc) can be (A) acheived with little - though not a total absence of - material goods; and (B) often focusing on consuming above this threshold brings tradeoffs with these lexically superior things. For example, getting stuck in a 'lifestyle trap' to sustain a high level of consumption (two cars, big house w/ high mortage, private school for kids), but as a result doing an evil/boring job and 'wasting' your life. How do we determine these important things? One example is to look at deathbed regrets - most people don't regret seeing their kids' birthdays instead of working overtime to fund a bigger car. This theme is repeated in a variety of movies and literature - writers seem to believe that accruing large quantities of material goods is not a satisfying narrative payout (comapred to say, love or 'the friends we made along the way')
  • Obviously there are counterarguments such as (1) Why prioritise deathbed you over current you? What gives them epistemic priority? and (2) who are you to tell me 'I am wasting my life!', and who chooses what is important? Further, it may be some people are genuinely fulfilled by buying large quantities of material goods. 
  • A response could be that (I) people often regret their lives in the moment, and consumerism has a way of trapping people in irrational - at least, according perhaps to a counterfactual version of themselves living a more fulfilling life - life paths; (2) There is no satisfying theoretical response to this. Parfit raises this problem with his 'muzak and potatoes' argument - that no quantity of pleasure from those items could outweigh the pleasure of Mozart's music. That said, it may be that the proportion of those trapped, according to (I), is actually quite high (either because of consciously experienced lifestyle traps or a lack of awareness/access of/to other lifestyles). The problem then of consumption culture is that it misleads people into not fulfilling some idealised rational version of themselves's values. 
• A creative argument (seen from the traditional right and the far left) that consumerist culture, defined above, makes it harder to produce great art/culture. The idea is that an excessive focus on mass-consumption, and profits from this, drives artists to make content for the lowest common denominator, and thus they no longer make 'transcendent' or 'great' art. This is where the term 'selling out' comes from, which is inextricable tied to consumerism. Naturally it also relates to how one assesses 'great' art (is there such a thing? There is at least new and innovative art which consumerism might hamper). 
• I should also note that many of these arguments also hinge on how one consumes. The wine-taster, for example, is apparently more morally appealing than the inveterate day-drinker. Similarly, their approach to comfort and convenience is complex. The more extreme view is that both these factors are effectively unnecessary for a good life (especially common amongst religions), and in fact distracting. The more moderate view is that they are valuable, but comfort and convenience above a certain point are (I) overrated, insofar as you get stuck on a hedonic treadmill, and (II) can prevent you experiencing other, valuable states (eg, going 'out of your comfort zone'). 

These are a few arguments, I've tried to focus on the ones I find most persuasive. Although I would like to reduce many of these to empirically testable propositions, I fear, in fact, the crux of the debate may hinge on how 'elitest' you are willing to be about ways of leading a fulfilling life. 

(There are also political arguments along the lines of 'it is bad for the environment', or 'it prevents a worker revolution by instilling fascination for trinkets etc', but I feel they do not get to the heart of consumerism insofar as they are defined by their consequences, rather than the intrinsic ills of consuming as a primary end.)

There are various different experiences that people have that they consider valuable.

I can read or consume a LessWrong post and consider that experience valuable. On the other hand, I might also write or produce a LessWrong post and consider that experience valuable. 

For every person, you can look at what percentage of their experiences that they value are consumption and what percentage of their experiences involve production. There are also other experiences like having a conversation with a friend, that are neither consumption nor production.

One way to define the term consumerism would be to say that if most experiences that people value are consumption experiences that's consumerism. 

The issue with a lot of consumerist experiences is that while they are nice, they don't feel deeply meaningful. It mostly feels more meaningful to write a LessWrong post than to read one, yet it's easier to read than to write.  If you look at metrics such as the number of close friends that the average American has, they seem to be going down.

When fewer people find the experience of tinkering and building new things intrinsically valuable and spent a lot of time with them, that's in turn bad for innovation overall in society. 

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As an aside, the interview discusses David Graeber's bullshit job thesis. David Graeber defines a bullshit job as a job where the person doing the job thinks it's bullshit. When I hear about environmental impact assessments taking years, I would expect that it's easy for the person writing the assessment to think it's bullshit. 

I have a friend who does some number crunching for creating ESG numbers for a bank and who sees his work as bullshit work. There are a lot of bureaucratic rules that create bullshit work, and for the progress movement it's important to reduce that bullshit work that rules like NEPA produce. 

From a progress perspective, David Graeber was much better than your average person on the left. If you haven't seen it the debate where David Graeber and Peter Thiel agree on most things about the Great Stagnation is good. 

I'm sympathetic to the idea that "Consumerism" might be too often used. But - with the risk of overlap with qjh's detailed answer:

Consumerism = (e.g.) when we consume stuff with very negligible benefit to ourselves, maybe even stuff we could ourselves easily admit is kind of pure nonsense if we thought a second about it, maybe driven by myopic short-term desire we'd ourselves not want to prioritize at all in hindsight. Consumption that nevertheless creates pollution or other harm to people, uses up resources that could otherwise contribute towards more important aims, resources we could have used to help poorer persons so easily. Things along such lines. And I have the impression such types of consumption are not rare - and they remain as sad a part of society after this post as ever before, no?

So I struggle to see what to learn from this post.