Pack people in tiny spaces like rats and tell them is good for them because urban density will save the world… and you get declining birth rates because having a child with no space having to work so many hours to keep up a marginally reasonable standard of living seems ridiculous to increasing numbers of people.

It’s natural feedback from overpopulation, which was matched with equally dire predictions which were wrong.

Right now your genes can’t get ahead by having lots of kids, unless you’re quite rich they will have disadvantages compared to kids with fewer siblings.

When the population starts declining you get the opposite pressure.

The thing to worry about is world economies based on expectations of infinite growth and the fallout when that starts being not true.