It is foolish to think that the consequentialist principles we use to assess the values of different populations could ever be the only principles in an acceptable moral theory. They have to be accompanied by supplementary principles setting constraints which we must not violate while pursuing our population goals and which we must not violate in particular by taking the lives of existing people. If we are to assess population principles as population principles, then we must assess them in circumstances where these constraints do not apply, that is, in circumstances where only increases and not decreases in the human population are in question.
This looks like the kind of mistake I had in mind when I wrote 'Anti-Consequentialism and Axiological Refinements': what Hurka interprets as a need to go beyond consequentialism, I see as a need to refine our axiology. If Hurka's right, then we should think something like the following: "Though it'd violate the moral rules to help bring about this outcome in any way, I must say it'd be really grand if all those happy folks of below average welfare would just drop dead. Here's hoping for some well-placed lightning strikes!"
But of course that's not what we think at all. It's not that their premature deaths are a good outcome that we simply aren't "allowed" to bring about. Rather, it'd be a bad outcome in its own right -- as we can see when we consider the scenario where the deaths are a result of natural causes. (Hurka's last sentence seems to neglect this possibility.)
Compare two very different forms of 'average utilitarianism':
(1) The value of a world is a function of the average happiness at each moment: e.g., the sum (or perhaps the average) of the momentary net happiness divided by the momentary population.
(2) The value of a world is a function (namely, the average) of the welfare values of each individual's whole life. (Welfare need not be temporally located.)
The "killing to promote average utility" objection only makes sense against the type-1, momentary view. On the second view, where we take a timeless perspective, killing someone does not reduce the (eternal) population. It merely makes one of the lives shorter than it otherwise would be. But that life still counts as one life in the history of the world, the same as it ever did. So, if death was bad for the person -- if it made their life worse than it otherwise would have been -- then, all else equal, it thereby reduces the average welfare of the world. It thus counts as a bad outcome. And doesn't this seem by far the more plausible view?
More generally: It makes a big difference if we understand individual welfare as a value that inheres in whole lives, rather than mere momentary timeslices. We've seen that it allows us to avoid the absurd result that harming some (by killing them), while helping no-one, could improve the world according to the average principle. It also helps against a related objection that applies even to the total principle: that killing someone and replacing them by someone slightly happier would increase utility. This may be true of chickens and other beings that lack a persisting identity, but you can't replace a person without cutting short a temporally-extended life, the disvalue of which might easily outweigh the increase in mere momentary happiness.