That's in theory. In practice the shock that ending that will cause will end any hope the US has of paying back it's debt, or even the interest on it, immediately. So it will cause an immediate default and destroy the economy that promised to pay it back in one fell swoop. There's no way out of that situation that doesn't involve reneging on repayment. Either directly (any sovereign nation, like the US, can simply legislate it's debt away like Greece -partially- did, and counterintuitively investors have no choice and will immediately reinvest, again, like Greece demonstrated)

So the US, like every other nation on this planet (barring a few, but only a few exceptions), has zero hope of ever paying back it's debt. So it won't.

That doesn't mean the illusion can't last a few more decades or even a century or two (though only a few states managed to make it last even a single century, the most famous being the Roman republic).

The reason that everybody's happy with this state of affairs is that it allows, at this moment, an economy that's factors larger than it would otherwise be.

This situation came to be as an "accident" (that has happened hundreds of times before in history), and there simply is no conceivable way out. Anybody who can rattle the boat risks losing big, even if it's monetary interests are protected, so the situation keeps existing until someone really, really goes too far.

Apres nous, le deluge. (google this sentence)