In absolute terms, the US is probably competitive as the most indebted entity in history. In relative terms, they are well past the threshold where countries pay back their debts in full. Their past performance isn't really a guide to what happens next, unless they've changed what is being measured the US is a financial basket case [0]. They aren't going to pay back what was lent to them in real terms, the idea is preposterous.
https://carnegieendowment.org/chinafinancialmarkets/86397 presents a reasonable argument for why you can't just compare debt-to-GDP between very different systems and expect it to make sense. (The US in particular is, of course, in many ways an outlier.)
so what? Have they missed an interest payment? Is their economy unproductive and cannot pay the interest?
Having the debt means you are entitled to a portion of the economic output of the machine behind that debt. Why is it a must for the debt to be repayable in one go? Esp. if the debt was not all issued in one go?
each time a particular treasury bond is due in full, the US treasury has been able to repay it. Whether they repay it by issuing new bonds, or increase their taxes and pay it off, makes no difference to the holder of that bond.