Once the rest of the world flees our debt auctions entirely, it will be mandated that your private 401k or pension invest, say 25% of its value, in government bonds. It will be sold as making sure the portfolio includes a portion of “safe” (no interest) US government bonds. Justice Roberts has already established that the federalism can compel your purchase of a service.
That will delay bankruptcy by a year or two, while destroying the 401k system and causing a backlash at the polls.
It's easier to continue monetizing debt, and lazy, timid politicians always choose the easy way out.