This is pedantic. Do you understand what "at the same time" means? By the time birth rates plummet below replacement rate in Africa, they would have risen well above replacement again in Europe and North America.

> 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?

We won't need to import labor from Africa when they have their own labor shortage because the birth rate in the US would have already recovered. The labor gap won't hit everywhere at the same time. That's why immigration works. In fact, Africa might need to import labor from America when the gap hits them.