They are "plummeting around the world at the same time," they just didn't start plummeting everywhere at the same time.
> The labor gap is a short-term problem and can be fixed with immigration and maybe AI. Birth rates aren't plummeting around the world at the same time. By the time the birth rate becomes a problem in Africa, America would have long recovered.
Except Africa can't fill the gap for everywhere else, and if immigration is the "solution" then what do you do about Africa after it's been sucked dry of its prime labor force? Just leave all its poor elderly to die on their own?