The reins of the world will be passed to whatever country successfully navigates clown world without experiencing a total demographic collapse. South Korea and Japan already tapped out (sorry K-Pop and Japanese cartoon fans), China is trying to join them by going from widespread infanticide of girls to creating a cyberpunk-esque dystopian nightmare society that nobody wants to live in.
Africa will continue to have way too many children, but they’ll probably die in some war before they turn 20, if disease and pestilence doesn’t get them first.
Europe and North America, both have problems but people are still having kids, less so in Europe. South America, no fucking clue.
Who will win? It’s anyone’s guess, but I can tell you that Japan and S. Korea are gone, best Korea can’t seem to stop widespread famines and misery so they’re out too. China is close, and the rest of the world isn’t far behind.