They're heavily invested in us, both in terms of bonds and in terms of selling us stuff. Both of that goes away the second they challenge us militarily, and their economy couldn't take it.
I'm not so sure their economy is still as weak as it once was. My impression is that China really has its shit together, more than any other country. Their economy isn't as big as the US yet, but it's catching up and maturing. I think the time will come when China will insist on dominating its own sphere of influence.