In the last 40 years, the 30 year treasury bond has gone for rates of less than 2% and more than 14%. The interest as a percent of tax revenue metric would have predicted the death of America in the 1980s, while oppositely making it look like things will be forever rosy today. Neither of those perspectives accurately characterizes the debt burden.
If the USA falls into a debt crisis the result will be global economic disruption.
If the world falls into runaway climate change the result will be the extinction of the human race and possibly all life on earth.
Personally I'm more comfortable with the possibility of the first than of the second.
If we could have runaway climate change somehow not disrupt global economy, we could eventually tech our way out of it. But the game over point is when the economy stops, and humanity no longer has capability to slow down and mitigate the effects of warming. At that point there isn't much left of humanity anyway.
E.g. how easy would it be for us to bootstrap back to space capabilities after a catastrophic war wiped out a large portion of world economic capability?
If something pulled the plug on our economy, all the technology we have today would disappear in few years to decades. Devices wearing out with no way of repairing them or producing new ones. We'd regress to medieval times, because that would be the most advanced things available energy sources would support.
The industrial revolution happened because we had easy access to high-density energy source in the form of coal. Maybe, given enough time, some survivors would figure out how to do a new industrial revolution without it, though I'm not having high hopes. So whatever is left of humanity would have to wait until the Earth recycles enough living matter to create new deposits of high-energy something.
Only if you can find people or institutions willing to buy the new bonds.
_if_ those cannot be found, the ‘easy’ (easy because all it’s debts are in US dollars) way out for the USA is to print money, but a country that has to print money because nobody wants to lend it money will find it hard to buy stuff from other countries.
Ultimately, what is and isn't possible for a state to do stems from the vagaries of what all other states are currently doing.
It's somewhat rooted in economic output, but there are a lot of other independent variables in the pot.
But you never really have to do that. You can just keep rolling them. If we could keep the debt at the same absolute level inflation and productivity increases would drive it into insignificance.
Of course the idea of keeping it at the same level is a fantasy.