Think about it, the nations of the world are now running fiat currencies, un-backed by gold or anything else. As a result of the 'financial crisis' the US, EU, China UK etc all had to go into some kind of QE to keep things from unwinding. All currencies are being inflated in a sort of sloppy but coordinated way. Thus parity is not in any real danger. The USD is not dropping out of sight, or crashing. Why did/does China lend to the US? Think back to the industrial expansion of China. China used the demand from US for cheap products to stimulate industrial expansion. This required that they get on the USD bandwagon. Once started, it is hard to get off. China is pushing currency swaps with other nations now, and this will gradually reduce the demand for USD as reserves over time. Not catastrophically, not drastically.
The US is rolling debt over. Not paying it off. It is the t-bill standard. In reality the debt is not being payed, because the central banks that are caught in the loop cannot do anything with the dollars and t-bills they have except recycle them with the US treasury. Central banks that have USD/t-bills cannot buy anything with them. The US has no intention of stopping the rolling over of treasuries. But it is not really paying off the debts. Again this is the 'fault of composition' thing. You can do something with USD if you have it, and buy something. The central banks that have USD cannot buy anything with them, except exchange them for t-bills and they can sell/swap USD for other currencies, and they are doing some of that. But purchase of US assets is forbidden. Only in special, select cases can anyone outside of US buy US assets. Buying consumer goods by individuals is completely different, and allowed.
"Superpower can rapidly become a 3rd world country - that happened to USSR. USA also are not guaranteed to last forever." And how is that going to happen?