It's important to remember that government debt doesn't ever need to be repaid, sort of how an immortal man could indefinitely refinance loans. That he'd have to borrow again to repay the old loans (while maintaining the debt) is part of the mechanism. Such an immortal could have his debt grow indefinitely and still be fine as long as lenders believe he'll be able to afford paying the interest and that other lenders will be willing to refinance the loans when they expire.

That's not to say that government debt couldn't become a problem, even a serious one, but as long as the economy grows fast enough to support the interest payments, it's not an "existential problem". The danger isn't so much the debt itself but in the confidence in the US economy falling below the level required to sustain that debt.

I loan the US government money by buying treasury bills because I trust that when they mature, others would be willing to lend the US money. When this trust in the health of the US economy declines, then there's a problem with the debt, but then there are also other big problems. What you'll see is rising interest rates that is likely combined with an unpromising economy, and yeah, that's a serious problem. A high debt could definitely exacerbate it (and that's why it's helpful to slow down the growth of the debt or even reduce it if possible), but it's not its cause.