This is often touted as a good thing, but the more I think of it it just seems like it isn’t “having such a good life in a developed economy” that makes people have fewer kids.
It seems more and more clear that increasingly competitive economic conditions are the reason people in the developed world have fewer kids.
The US had sustainable birth rates throughout the 1990s to 2008. Not exactly the Stone Age. But they began a consistent downward trend since then.