I want to thank Irgy for this idea.
As people generally know, total utilitarianism leads to the repugnant conclusion - the idea that no matter how great a universe X would be, filled without trillions of ultimately happy people having ultimately meaningful lives filled with adventure and joy, there is another universe Y which is better - and that is filled with nothing but dull, boring people whose quasi-empty and repetitive lives are just one tiny iota above being too miserable to endure. But since the second universe is much bigger than the first, it comes out on top. Not only in that if we had Y it would be immoral to move to X (which is perfectly respectable, as doing so might involve killing a lot of people, or at least allowing a lot of people to die). But in that, if we planned for our future world now, we would desperately want to bring Y into existence rather than X - and could run great costs or great risks to do so. And if we were in world X, we must at all costs move to Y, making all current people much more miserable as we do so.
The repugnant conclusion is the main reason I reject total utilitarianism (the other one being that total utilitarianism sees no problem with painlessly killing someone by surprise, as long as you also gave birth to someone else of equal happiness). But the repugnant conclusion can emerge from many other population ethics as well. If adding more people of slightly less happiness than the average is always a bonus ("mere addition"), and if equalising happiness is never a penalty, then you get the repugnant conclusion (caveat: there are some subtleties to do with infinite series).
But repugnant conclusions reached in that way may not be so repugnant, in practice. Let S be a system of population ethics that accepts the repugnant conclusion, due to the argument above. S may indeed conclude that the big world Y is better than the super-human world X. But S need not conclude that Y is the best world we can build, given any fixed and finite amount of resources. Total utilitarianism is indifferent to having a world with half the population and twice the happiness. But S need to be indifferent to that - it may much prefer the twice-happiness world. Instead of the world Y, it may prefer to reallocate resources to instead achieve the world X', which has the same average happiness as X but is slightly larger.
Of course, since it accepts the repugnant conclusion, there will be a barely-worth-living world Y' which it prefers to X'. But then it might prefer reallocating the resources of Y' to the happy world X'', and so on.
This is not an argument for efficiency of resource allocation: even if it's four times as hard to get people twice as happy, S can still want to do so. You can accept the repugnant conclusion and still want to reallocate any fixed amount of resources towards low population and extreme happiness.
It's always best to have some examples, so here is one: an S whose value is the product of average agent happiness times the logarithm of population size.
There's a simple way to kill the RC once and for all: Reject the Mere Addition Principle. That's what I did, and what I think most people do intuitively.
To elaborate, there is an argument, that is basically sound, that if you accept the following two principles, you must accept the RC:
Mere Addtion: Adding a new life of positive welfare without impacting the welfare of other people always makes the world better.
Nonantiegalitarianism: Redistributing welfare so that those with the least amount of welfare get more than they had before is a good thing, providing their gains outweigh the losses of those it was redistributed from.
These two principles get you to the RC by enabling the Mere Addition Paradox. You add some people with very low welfare to a high welfare world. You redistribute welfare to the low welfare people. They get 1.01 units of welfare for every 1 unit taken from the high welfare people. Repeat until you get the RC.
So we need to reject one of these principles to avoid the RC. Which one? Nonantiegalitarianism seems unplesant to reject. Pretty much everyone believe in charity to some extent, that it's good to give up some of your welfare if it helps someone else who needs it more. So we should probably reject Mere Addition. We need to acknowledge that sometimes adding people to the world makes it worse.
Does rejecting Mere Addition generate any counterintuitive results? I would argue that at first it seems to, but actually it doesn't.
A philosopher named Arrenhius argued that rejecting Mere Addition leads to the Sadistic Conclusion. Basically, he argues that if adding lives of positive welfare can be bad, then it might be better to add a life of negative welfare than a huge amount of lives with positive welfare.
This seems bad, at first, but then I got to thinking. Arrhenius' Sadistic Conclusion was actually a special case of a larger, more general principle, namely that if it is bad to add lives of positive welfare, it might be better to do some other bad thing to prevent lives from being added. Adding a life of negative welfare is an especially nasty example, but there are other examples, people could harm themselves to avoid having children, or spend money on ways to prevent having children instead of on fun stuff.
Do people in fact do that? Do they harm themselves to avoid having children? All the time! People buy condoms instead of candy, and have surgeries performed to sterilize themselves! And they don't do this for purely selfish reasons, most people seem to think that they have a moral duty to not have children unless they can provide them with a certain level of care.
The reason the Sadistic Conclusion seems counterintuitive at first is that Arrhenius used an especially nasty, vivid example. It's the equivalent of someone using the Transplant case to argue against the conclusions of the trolley problem. Let's put it another way: Imagine that you could somehow compensate all the billions of people on Earth for the pain and suffering they undergo, and the opportunities for happiness they forgo, to make sure they don't have children. All you'd have to do is create on person with a total lifetime welfare of -0.01. That doesn't seem any less reasonable to me than tolerating a certain amount of car crashes so everyone benefits from transportation.
So we should accept the Sadistic Conclusion and reject Mere Addition.
What should we replace it with Mere Addition with? I think some general principle that a small population consisting of people with high welfare per capita is better than one with a large population with a low level of welfare per capita, even if the total amount of welfare in the larger population is greater. That's not very specific, I know, but it's on the right track. And it rejects both the Repugnant Conclusion, and the Kill and Replace Conclusion.
Really like your phrasing, here! I may reuse similar formulations in the future, if that's ok.