If you look at the reasons why people are having fewer kids

They don’t want to, and now have contraceptives? It’s a pretty across-the-board phenomenon, there’s no reason to think a different time would change it any more than a different place. Developing countries have dropping birth rates, collapsing countries have dropping birth rates; as do both rich and poor.

Throw around “absurd” all you want, you’re not an expert demographer either.