China and Russia can wall themselves off and survive. We can’t.
The British Empire became a shadow of its former self in just a few decades. Once gone, the glory did not come back. If America falls apart, then we will most likely not get a second chance. Our demographics, our education, our work ethic, our sense of morality will be gone, and we will have no foundation upon which to rebuild.
If China decides that we are not worth lending to, then we are in trouble. Sure, China would feel some pain too -- but they could "wall themselves off" and they'd be OK. We, on the other hand, would cease to be a power to be reckoned with. In just a few decades, the world would be totally dominated by China. I think they know that. I think they might even see our demise as a good thing, even of they lost all the money we owe them.