Population ethics is the philosophical study of the ethical problems arising when our actions affect who is born and how many people are born in the future (see Wikipedia here).
In the example I gave we are judging the ethical permissibility of a change in population (one extra life) taking into account the welfare of the new person. You implied it can sometimes not be ethically permissible to bring into existence an additional life, if that life is of a poor enough quality. This quite clearly seems to me to be engaging in population ethics.
You say:
World-states are assigned value without considering, to whom and for what?
This isn’t usually true. In population ethics it is most common to assign value to world states based on the wellbeing of the individuals that inhabit that world state. So in the case I gave, one might say World A, which has a single tortured life with unimaginable suffering, is worse than World B, which has no lives and therefore no suffering at all. This isn’t very abstract—it’s what we already implicitly agreed on in our previous comments.
What assumptions are these specifically?
You’re welcome to lay out your own theory of population ethics. The more I read about it though the more it seems like a minefield where no theory seems to evade counterintuitive/repungnant results.