It seems to me this is about accounting and how things are enumerated, not reality. That's why I look at total debt, not "debt held by the public". And of course if nothing is done with the SSTF, that contribution will disappear by 2035 or so, so we'd be back to zero, except the Fed.
If the Fed hypothetically assumed ALL the public debt, they could zero it out, but in my view, the total debt would still exist and be horrible. It's not as if that would make everything kosher.
My GUESS is at some point, the Fed will somewhat quietly have to assume more and more of the debt because lenders will become more and more worried and the T bills won't be viewed as the safest investment in the world. Then we could experience really serious inflation of course, which would increase interest rates, which would contribute to really really serious inflation ...
The unfettered ability to "print money" is scary, to me.