I think the default mode is lack of population and getting outcompeted by pro-natal civilisations, like amish etc. You need to provide a reputable source for why is that not the case.

I must be either misreading your point or not recognizing intentional irony. I would expect the Amish to average a lot of kids per family, but I'm not aware of any instance of a civilization disappearing due to being outcompeted by them.