Just tell me what you want, and I'll comply.

If you prefer not having disagreement, I'll just have to read and not participate. I can't simply agree because others think differently. Surely that is not what this is about.

I'm generally against ban-level measures, as such measures are very damaging and the comments don't seem particularly so.

This is the first time I've come across the suggestion to downvote well-thought-out contributions to silly conversations, actually, and I like it. I'll keep that in mind.

my recent attempts to discourage those who make high-quality contributions to hopeless or malignant conversations didn't stir much enthusiasm

FWIW, you brought me around on this point.

my recent attempts to discourage those who make high-quality contributions to hopeless or malignant conversations didn't stir much enthusiasm

I generally regard it as a norm that one should not respond to trolls and the like.

But I ignore that norm when I see the opportunity to help someone.

A specific suggestion I have is to establish a community norm of downvoting those participating in hopeless conversations, even if their contributions are high-quality

How do you define "hopeless", exactly ? Sometimes, a high-quality post in response to a troll's or idiot's thread can be quite helpful to other readers (and lurkers), who aren't trolls or idiots, but who are just misinformed or new to the topic. I personally have been such a lurker on several sites, but I do acknowledge that my personal experience is not statistically significant.

There is a point at which not getting the message from karma is sufficiently damaging to the community that moderator action is called for.

The question isn't whether it "exists in order to" make cracking down unnecessary, or whether it "is supposed to" replace moderator action. The question is whether it actually does those things. And it's far from perfect at doing them. Yes, heavily downvoted comments take up a little less space in the recent comments and in the thread (at least if you have the willpower not to click on them! virtue of curiosity!) But they still take up some space; they take time to be downvoted enough to be hidden; I'm pretty sure they still appear in the sidebar; and the responses to them tend to appear in full, even though these too tend to be valueless. On a more abstract level, I'm worried that such comments influence a collective sense of what the current topic of the site is.

There are intellectual problems other than arguing for the wrong views, and ways of being ban-worthy other than being a troll or spammer. I haven't read most of the exchanges, but it was certainly my impression that Monkeymind has been communicating in ways that downvotes had made very clear weren't working for the audience, that he's been reasoning badly, and that he's been responding with hostility to downvotes. Are you sure that nobody has been banned for such behavior previously, and that a genuine Rubicon is being crossed here?

If the current system is so perfect that the comments being banned weren't attracting any attention anyway, is it really a big additional problem for them to be censored?