Yep, brain drain.
Even though world fertility levels will fall, do bear in mind for all those current kids pairing up when they're adults, will still add on one or two kids on average, and their kids in turn will have one, all in the same lifetime.
This is why places such as India which has seen fertlility plummet to below replacement levels will still add on 400 million to its population through the 'decline'. Africa has not peaked (it's doing so now), even though its birthrates have also plummeted they're well above replacement still at above 3-4 kids per couple. Every continent in the world will be static or declining this century except for Africa. Note how North America and Europe will stay static despite low birthrates, thanks to immigration:
^Africa will likely become the centre for world population in the next century, Nigeria alone may even be approaching China's population (highest estimates at 930 million) but with a far higher amount of working age adults. The world's largest singular cities will also be in Africa: