Gambit you did make some fair points. But i beg to differ with you on some as well.
The U.S didnt just emerger as a world superpower/power through internal consumption/westward expansion alone(though it was a major part of it). The U.S had a great depression as well before the second world war, In fact the U.S was also quite Lucky the second world war helped propelled the U.S economy/manufacturing. After the destruction of europe/Asia. the U.S was the only major power who was not affected/destroyed by the war. So it emerged from the war from being a limited regional power to a superpower overnight, and took over from a war ravaged/tired britain. The second world war did help the U.S alot both economically(as it loaned cash to several countries destroyed by the war).
Moreover, China growth is mostly investment driven than though FDI or export nowadays. So if any argument there should be is that the Investment is not sustainable. However their demand is also increasing, and right now they are the second largest importer in the world after the U.S and are set to overtake the U.S less than 3 years from now. So its kind of difficult to look at the country froma simplistic view, ince its more complex than that. Though their demand is still well below their potential or what it should be, but nonetheless its big and will keep increasing.
The advantage China has is that, its still despite its miraculous growth these past decades a poor country, and still has hundreds of millions of people whose demand/capital etc is only set to increase with each pasing year. They still have a large room for growth, and have not yet even reach half of their true potential. This is the only reason why i believe they will overtake the U.S in the cominf decades from now. Its not because the communist party is efficeint or a genuise or whatever(which i think they arent by the way, since i believe had the nationalist won, China will have been open long ago and way way more bigger in all aspect than it is today), its just that they are coming from a low base down, and when you come from such a base , you are only going to go up, given you have at least a decent/average administration.
Even if they acheive just half of what Japan has acheived(productivity/gdp per capital wise) then they will still be twice as big as the U.S economically.
The U.S shouldnt feel bad/uneasy if.when it is overtaken by china, since the U.S will still remain the most influential country in the world for a long time to come, be it culturally, technologically/scientifically etc. This is something that will take China or any other potential power a century/decades to achieve despite if they are bigger than the U.S or not.