> And anyway, we have a precedent for what happens when there is significant population reduction: the Black Death. Life got way better for the vast majority of people who survived that event.
The difference between the Black Death and a population decline driven by lower fertility rates is that the Black Death disproportionately claimed the lives of the old and the frail. The result of that (at terrible cost) was an increase in average productivity by removing the people who would otherwise have required assistance, along with a reduction in competition for scarce resources (like food).
A population decline driven by a reduction in the fertility rate effectively does the opposite -- you end up with more elderly people compared to the number of young working people. And we haven't had a meaningful scarcity of food in centuries, so the things that are scarce now are certain types of skilled labor (e.g. doctors), which you can only really source from young people because there aren't a lot of medical school applicants from the population of people 55 and older.