The US wants to maintain positive inflation, and much of that debt is below the targeted inflation rate which offsets any concerns about the need to create 31$T in new money. Aka if they can pay back 31$T by borrowing 30$T inflation adjusted and then pay back that 30$T by borrowing etc, it’s just free money.

The risk isn’t the need to come up with 31$T at some point the risk is people being unwilling to lend close to the inflation rate. The US economy is addicted to a subsidy from foreign borrowers, and that changing could be seriously problematic.