They didn't collapse for a few millennia and now they would? China is still very underdeveloped country, so they would just get back where they started three decades ago. Not a long time really, taking into account the length of their history...
Yes, they are finished. Indebted already as grotesk as the globalized developed just-in-time-world, on which their economy is depending.
What do you mean by being finished actually? Through all their long history China has always been very closed society. What would change so dramatically in the life of ordinary Chinese people if the United States were to crash? They would just get to their villages and back to rice fields where their ancestors had been toiling for centuries...