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).
[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.
It doesn't matter what the rules say right now, his wealth is at risk. They can and do change the rules sometimes. They can change what the word "inflation" means. Maybe someone legislates that inflation is 1% now for paying back I-bonds, I dunno. This is risk, it could happen. Something is going to happen, the debt situation is unprecedented bad. Will that something affect these bonds? It is possible.