Don't see how that could happen, as other countries would still hold the individual states responsible for their state debt, and all the new countries responsible for the former US national debt as successor states.And likely the US will dissolve to a more tyrannical form of government like national socialism, theocracy, or fascism.
Also a western brain drain would result with the entrepreneurs and well educated middle class fleeing the US for Asia and never coming back.
There's no evidence of any of that.
The several smaller nations that would form would manage to make do without oppressive fed gvmt central planning.
That's how this nation was formed in the first place. secession from an oppressive central gvmt.
They can "hold" whatever they like...When this nation goes through the inevitable default/collapse/partitioning/rebuilding, old debts won't matter very much.
No one is going to do anything about it...