The domestic bondholders would pay the inflation tax and are all wealthy enough to have savings, ie, they are also taxpayers. They're not getting out of this no matter what happens; they're going to lose out.

The question is really do the bondholders directly eat the losses (20% foreigners) or US consumers and their tax base eats them (potentially ~0% foreigners although inflation is so complicated in practice it is hard to tell). It'd be pretty easy to handle the situation by a standard default and probably better for the voting base. The risk of the politicians breaking that way is obviously present.

It is risk, it doesn't even have to be likely. It is just the question of "will the bondholder get all the money they're promised?" and the answer can obviously be no. The US can't afford to repay their bonds, purposefully triggering vast inflation would be quite damaging to the US political class and there is a chance they decide not to. It isn't a negligible chance.