The last point is all that matters. It will be painful and take a while for Americans to come to grips with it, or we will all die in an apocalypse when they can't accept it.
Economically, demographics are insanely against China and it is not clear that immigration is a viable way for them to close that gap.
Finally, their unique political system has not been tested during a period of low growth. It is not clear they have the political stability to be a reserve currency. Again, the US has been pushing their luck.