[1] https://www.bls.gov/cpi/additional-resources/historical-chan...
It isn't a matter of credibility at this point, it is one of political realities. There hasn't been a plausible outcome where the debt principle gets paid back since the 1970s, but that turned out to be OK because they can keep rolling that over with new debt. Alright. Well now we're approaching debt levels where even paying back market interest rates is coming under pressure and probably going to give.
The risks here, much like the debt to GDP ratio, are starting to move off the charts. Lending the US government money can't possible be safe under these conditions. Someone here is obviously going to get screwed, some of these promises have to be broken. It could just as easily be these bonds as anything else.
Of course it _could_ happen - nothing prevents it. And if they did, they'd have destroyed the credibility that took over a hundred years to create, and it would remove any future possibility of people lending money to the US gov't. A rational actor would not undertake this action, unless there's some good reason (such as nuclear war).