Japan grew rich before it grew old. China is going to skip the first part and plunge straight into the second.
There are more "rich" in the PRC in recent years than there were in Japan's heyday.
People, especially Americans, get lost when it comes to the scales of the world.
Japanese society reinforces a flattening of society, which means even poor Japanese get to share in a tiny bit of the wealth.
And trying to damn the PRC by appeals to "demographics" is silly because all modernized socieities in the world are below replacement rate. Only those areas still (so-called) developing have significant population growth.
120M is too many people for the Japanese islands to sustain.
1.2B is too many people for the PRC (and its conquered territories) to sustain.
1.3B+ is too many people for India to sustain, and if one throws in Pakistan and Bangladesh there are way too many people for south Asia to sustain.
And so on.
We're going to have to reckon with the new world of slow population decline in many parts of the world.
North America could hold some more people, but the xenophobes, who happen to be the same people supporting Trump's foolishness, don't want to accept people with too much melanin in their skin.