The point is Japan is stagnating. Between 1980 and 2020 Japan averaged 0.41% GDP growth per year. Meanwhile most of the world did much better. In relative terms, Japan was a far more important player in 1980 than it is today.

It may be hard to believe, but the same will inevitably happen to China, and it's near-impossible to change because the damage has already been done. It's the inevitable legacy of enforcing Deng Xiaoping's One Child Policy for 35 years.

The same might apply to China but by then they'll have a far larger economy than the US's.

What should not be forgotten is that Japan's population is only 1/3 of the US's, that it has extremely limited land resources, and that it is very developed (see GDP per capita). None of these apply to China.

If China had the same GDP per capita as Japan then its economy would be 12x larger than Japan's and 2.6x times larger than the US's.

China is still relatively poor per capita and thus still has a huge margin before reaching a peak, even if breakneck growth cannot be maintained.

Having an aging economy didn't stop Japan from becoming the world's most advanced economy (based on economic complexity: https://oec.world/en/profile/country/jpn).