China ironically can end US by simply providing easy immigration for qualified tech workers.
I dont believe those tech workers would wish to move, unless the political system in china changes to one that is more amenable to democracy; not to mention that having high salaries in the US, it will be impossible to achieve similar levels in china after migration (even if the PPP remains the same!).
China won't even have to have high salaries, all they would need to do is basically set up immigrant neighborhoods that have all the familiar things that US people like, and through the nature of just being around people of similar status, whatever the salary everyone gets paid gets normalized - there isn't anything you would be able to buy to "flex" on your peers, and everyone would be in the same boat.
Democracy isn't meant to give you the best outcome. It gives you, the voter, a way to push the gov't.
So far US has gotten measurably worse, while China has gotten measurably better.