No matter how sustainable humans live, there is a limit of a maxium number of humans that is reasonable for the plant and humans themselves. I believe this number has long been surpassed: just go in any larger city on the plant and it looks catastrophic. Large cities come with large slums, dirt, narrow appartments. Good number might be 100 million humans on earth, not 10 billion.

Population collapse? Why not?

Btw, you forget the future risk of pandemics. They could wipe a lot of humans.