If #Biden were to try to issue additional debt without legal authorization, future presidents would be under no obligation to recognize those claims. In fact, it would be arguably illegal for them to pay out of the Treasury to retire such securities.
But also consider the complications of circulating a bunch of invalid notes alongside valid ones.
For example, a pension fund might report that it holds a million dollars in treasuries, but some of that is invalid... which part of it? How would auditing and public disclosure deal with that?
People pushing Biden to just unilaterally issue more debt miss how complicated that would be.