There's some intersection point between long term decreasing in China's ability (demographic collapse) and long term increase in China's ability (their current build up of military hardware in air, land, and sea that is currently outpacing America's). Maybe somewhere in 10-20 years where their regional military power is much higher than America can project across the Atlantic but they still have a lot of military aged men.

Atlantic? IDK if China even has aspirations to play World Police like the US. Military protection of things like their interests and the stability of Belt and Road, sure, but I don’t see China trying something like the Gulf War or OEF.

It’s very possible that they will be able to dominate South China Sea and their zone of the Pacific, even now, given the proximity advantages and ship/missile production; and I think that would be satisfactory to them.

20 years from now, China’s sphere and America’s sphere are separate, with China having a lead in competing for Africa, and Europe in a very weird place socially, economically, demographically, and WRT Russia/US competition.