Eh, a terrible collapse requires them not being able to get something their currently getting. Generally, that requires not being able to buy things. Thus, the fact that they're no where close to not being able to buy things is important: the USSR for example famously was dealt serious death blows because their oil industry stagnated late 70s staying flat for 10 years while oil prices collapsed.
That above represents a real revue loss for the USSR from 1980 to 1990 while they kept the production level from $540 million at the peak to $180 million a day. Even going from the $50 per barrel to $30 per barrel represents a loss in income of $7 billion dollars of loss revenue per year. That's a single point that represents something like 10% reduction in USSR foreign sales, which were in the about $100-200 billion range (1990s dollars, so not quite the same).
I don't see anything that difficult for China in any of the data I've been shown. They are still building. They're population is still growing. There's economics problems which may get very bad . . . 40-80 years from now.
I don't know. All the data I've been shown on how collapse is immanent would have suggested that the Germans or English should have collapsed in 1980s or 1880s respectively. Which, obviously, did not occur. Now, Britain did fall apart as a global power in the wake of a global war, and a big global war could be bad for them, certainly.
Short of that however? Well, there's a Billion Chinese, and the factories exist at this point. They're still at half the GDP per Capita of Japan, so just competently copying Japanese efficiency still leaves room for about a doubling of GDP without anything else without having to do any reinventing of the wheel. They could stop growing in PPP terms and it would take the US 10 years of about 2% growth to catch up to where they are now. Anything that happens to china They're going to be in the top 3-4 States on Earth.