> We know that better living conditions (health, income, education etc) lead to lower fertility. In a world that you have both developed and developing countries, the stable equilibrium seems to be world suffering.

Alternately: in the past, dying was a lot easier, and society adapted to that by creating extra people, and we've reached a point where that isn't as necessary. That doesn't mean it wouldn't be awesome to improve things for the number of people we do have, or that those improvements are easy, but it's not obvious to me why the assumption would be that quality of life only changes if the population continues changing. In other words, it sounds like you're measuring two different things, noticing one of them slowing no longer increasing, and trying to make inferences about the other one without actually establishing how exactly that connection works.