We only have one modern example of demographic crash to study... Japan.
Total economic stagnation, spiraling population numbers, loss of anything anchored to manual labor, and an aging population.
So far, it looks to be a way for slow extinction of a culture.
Ah yes, the hellscape that is Japan. The Japanese are currently fleeing to South Sudan and Darfur in hopes of a better life.
I mean, if Japan is the world's future, isn't that kind of... good? It's not the first place I think of when I think of a failing country.
Japan, a country famous for no one knowing what its culture is.