It's well established that people (nations and groups within nations) have fewer children as their circumstances improve. Fixing circumstances will lead to fewer children, not more.

This has only been true generally in the West since ~1800 or so. Charles Darwin had fewer grandchildren than children. It’s also not universally true even in the West. Sub populations with higher fertility for the wealthy exist, Mormons and Hasidim for two. Culture beats biology, see Hutterites and the Amish in the US.

> Better circumstances are responsible for low fertility rates, not bad circumstances.

That depends on the definition of "better". In purely material terms, the developed world is the best there ever was. Yet depression, drug abuse, loneliness etc are rising, so maybe humans require more than just material things.

For animals, living in a zoo is the best thing to do. Steady meals, a doctor on call and they'll even bring you mates to reproduce. Yet somehow, many animals in captivity don't want to breed, maybe it's similar for humans.