the_dunk_tank

Population control at 10 or 15 billion people would have all the same problems as population control at 1 or 8 billion people

10 billion is fine, but in general, a more populated world (let's say 20 billion) would have way more problems, such as

1. way less biomass, due to fewer forests bc of the greater amount of open space needed for farmland
2. less biodiversity, as various animal species niches are taken up by more humans
3. worse climate because forests moderate both cold and hot climates (but especially hot)
4. worsening diet quality, increased reliance on grain and much less consumption of both meats and "inefficient" plant foods (like nuts, fruits, and vegetables)

Earth has a limit, our current population is perfect under a global socialist system, and even a bit bigger could be fine, but the margin between the human population and the carrying capacity is a good "buffer zone" should any catastrophes occur. You have to draw the line somewhere and I'd rather have a bigger buffer zone, people aren't gonna suffer because they can only have 2 kids instead of 3 kids

You could also hypothetically have systems that reward childlessness, etc. Not everyone has to have the same exact lifestyle, it's just that the global population should be stable.