The US is paying back with inflated dollars, meaning the dollars paid back are worth less.
Another possibility for the future, the US could always just default.
What would China do?
US will never default, they will just inflate the $ if they have to.
The US is actually more likely to raise taxes if it is unable to borrow to cover it's interest payments to creditors via debt refinancing (the same with when debt starts to mature). Credit will not dry up for the US government overnight so it will become apparent that it will need to make fiscal adjustments.
Also if the US were to default I would say it would be a selective default on only foreign government holders of treasuries
