Will it ever be paid? Probably not, but if the US GDP keeps growing, it can make the debt smaller as a percentage of GDP, which will make it less significant over the time.
The GDP is a flawed metric that can be manipulated.
It doesn't matter what the number says, what matters is the rate of taxes the government must levee in order to pay for the debt service and/or the debt. If the tax rates stay the same and tax revenue goes up, that means the GDP grew. Absolute numbers don't matter in macroeconomics, only percentages. If our debt size reduces as a percentage of GDP, then it gets easier to pay.
That doesn't mean this whole plan is always going to work, but that's the theory and thus far investors seem to believe the US will pay their debts, which is why the interest that the US pays is so low.
The nice thing here is that none of this is going to sneak up on anybody because the markets are forward-looking. Last year when Trump's actions threatened to quickly and severely wreck the economy, those interest rates creeped up a tiny amount, but it was enough to demonstrate to Trump who was boss, and he backed down. Trump is arguably the most powerful president in US history and he acts with absolutely impunity, and so if this structure constrains Trump, it's likely to constrain any president or future US government. (That's all assuming we continue to have real elections in the US and thus continued political pressure. If Trump rigs the election this year then the US will collapse for this very reason since the system keeping things in line will have disappeared).
Don't get me wrong, I agree that, politically speaking, we'll always have a debt since the population won't accept the higher taxes / lower spending until they absolutely have to, so we'll always run the economy right at the edge. And it's certainly possible that an incompetent / corrupt president will mess this up. There's a case for both a collapse and also continued stability though.
