As long as there's someone to produce it ... forever?

I somehow keep eating 6-10 eggs a week despite not having a chicken.

Growth. Same reason we never need the national debt to be smaller than it is.

I think people look at "national debt" as though we're buying trillions of dollars of stuff and not, fundamentally, two things:

1) mandatory saving for future, like a Social Security for the country (which is ironically also comprised of Social Security)

2) investments made into the future of the country. People buy T-bonds because they're reliable returns. Low risk, relatively low reward. If suddenly our future looks less bright, our debt will slow, but it will be a pretty telling canary in the coal mine.

It's baffling to me that people generally don't grasp this. They treat it as though we're just ... buying stuff on a credit card.

Like this goes back to Adam Smith; these questions have been answered for a long time now