"How were those bases established, pray tell?"
Yes - let's 'go there'.
The difference between having a strategic relationship with the US, versus one with China, is literally embodied in the difference between North and south Korea.
The US defeated a pariah nation, Japan, and helped build it into a highly advanced, free nation, with whom the US enjoys a very positive strategic relationship. And FYI Japanese could ask the US to leave at any time, but that won't happen because of course Japan absolutely wants to maintain their partnership.
North Korea is the ugliest, totalitarian 'prison state' in the worst in the world, maybe one of the worst the world has ever seen. That's 'Team China'.
"It would be monumentally stupid for the Chinese to just assume that we'll faithfully honour "international waters neutrality" forever, especially at the moment when they're eclipsing us economically."
Not only is this wrong, it also does not justify their actions.
First - so long as there is no 'hot war' the Chinese will be able to ship their goods and navigate waters as they please. It won't matter if they have a 'bigger economy' than the US.
Second - none of this justifies grabbing territory. As I mentioned, if China were to use its navy to actively support open navigation of international waters, then the world would welcome that. We'd all be working with on the issue. But that is not the case: China has Imperial ambitions, and if it means putting a few more million Muslims in concentration camps, then so be it.
It's really sad to see these constant duplicitous China arguments. That it's not perfectly black and white does not make it entirely murky either - China is a pariah state.