In recent discussion Patrick Kaczmarek informs me I’m absolutely mistaken to think it can problem with decision theory and helpfully suggested the issue might be the bridging principle between one’s axiology and one’s decisions theory.
The problem seems essentially the same as Parfit’s Hitchhiker: you must pre-commit to win, but you know that when the time comes to pay/spend, you’ll want to change your mind.
We can make necessitarianism asymmetric: only people who will necessarily exist OR would have negative utility (or less than the average/median utility, etc.) count.
Some prioritarian views, which also introduce some kind of asymmetry between good and bad, might also work.