If we're even competing on demographics in 2100, this means that China has become the global hegemon by ≈2050s, because there's been no ASI Wunderwaffe, and their industrial/robotic scaling eventually made American military power a rounding error
In 1972, nearly 8 times as many children were born in China as in the U.S. In 2025, only 2.2 times as many. Consider also that China’s total fertility rate in 2025 was 0.93, while the U.S. rate was 1.58.
If fertility rates remain at those levels, the U.S. will overtake China in births at some point in the 2060s (depending on immigration flows to the U.S.) and in total population by around 2100. A world where the U.S. has a larger population than China is very different from the world today.
Of course, this is a big “if,” but it is a nice thought experiment to illustrate the importance of demographics in shaping Great Power competition. Among the great powers, the U.S. is the least demographically exposed.