Imagine this, you're just sleeping on your bed all day but you're debt on the government is not sleeping instead it keeps on rising every second. Such an unfair situation in life of every single person.
You can even say that our unborn children will carry the burden as well. On the upside, if government borrows to fund infrastructure projects it will likely benefited the next generation.
And what when the country is unable to pay its debt? Nothing.
Yes, but most of the times you will hear nations, raising tax and the first argument they gave us is that they need to pay "our" national debt.
If the Chinese come knocking asking for the repayment of the said debt, see how the government try to become hostile and engage in a war (hopefully not) with China.
Never in the history of humanity we have seen that countries become hostile against each other because of not re-paying their debt. So I don't think that China and USA will go to that stage, they might be in sort of trade war but I'm sure they know the consequences of their action specially if China force USA for re-payment.