Agreed on Russia - but their collapse started immediately post WWI with forced collectivization. Being forced to live in multi-family apartments (or just single rooms) does not encourage childbearing. It got worse with WWII and the Holodomor (deliberate death by starvation).

That said, I'd agree that population collapse would be a disaster, but for far different reasons. Look at the way the area evacuated for the Chernobyl disaster has rebounded without people. The world can get along fine without us.

That said, we're about to find out how well people do with moderate negative growth. The size of the population was relatively stable for millennia until things like sanitation and then antibiotics were developed. Getting back to that will likely be painful (notably our economics are based on growth, as is the social contract for the young and middle aged to care for the old). But I expect we'll be fine - so long as the nuclear armed nations undergoing depopulation (RF and China) don't go out with a bang. Aaaand that's what I'm concerned about...

BTW, Peter Zeihan has a pithy explanation for the drop in fertility (paraphrasing): “On a farm, children are free labor; in a city, they're expensive conversation pieces…”

Finally, there are places *not* in demographic collapse (the US, France, NZ, Aus - even the Eastern oblasts of the RF). Again, we'll be fine... probably.