Back before tv and streaming and the internet what the hell did people do? Well it is becoming clearer every single day. They went all in for sex and had children. It seems that all of the civilized countries have fallen for netflix or roku and no longer get together to have exciting stimulating or even boring sex. The numbers don't lie. Most of the West isn't even close to breeding a new generation and none of them care.
That's kind of a shocker. Things will right themselves in time but I now understand how we 'rediscover' ancient cities or how Rome was abandoned. There is an ancient joke of some truth that says merely that the future belongs to those who show up for it. I read 'A Distant Mirror' long before I found girls interesting. It was on one of the 32 bookshelves in my parents house back when I was growing up. It described depopulation in a way that brings it home.
One of the many reasons cities are struggling is that they don't attract new subjects, punish the ones they have and don't foster new generations. Replacing the population with immigrants is a road well traveled. I'm not aware of any civilization that did that that survived. Say what you will, Rome is not populated by Romans and Greece is not populated by Athenians or Spartans. The Gauls and those savages that lived on the other side of the Rhine are not there anymore.
On the gripping hand I'm reading a book that makes reference to a destructive race that was wiped out by humans 85 million years in the future for practicing bad art.