If China suffers from the Middle Income Trap it will be the first East Asian country to do so. I believe in the CPC’s potential to mismanage the economy but I don’t believe that hard. More than likely we’re talking US levels of idiocy, stopping the high productivity cities from growing, not India or Argentina levels ever you just piss away entire decades with zero or negative growth.

For someone who only passed through a Chinese airport, what differentiates China in 2008 and 2020?

Why do those demographics matter, though? I can't imagine China having a strong welfare state or super expensive healthcare for the elderly. So old people won't cost China much (though they'll probably have miserable lives if they didn't save enough).

Even the population going down isn't really an issue, they have more poor people to take out of poverty than the entire US population, so more than enough labor force. Plus if their population declined 50% (a super far fetched scenario that could happen over an entire century), they will still have 2x the US population.

For most individuals living in China will still be mediocre to bad, but that's still enough for them to create in aggregate an economy 2x the US one.

Japan just couldn't do this, they were always smaller than the US.