The Fed will buy the bonds if nobody else will.
Of course, before we get there interest rates will go over 10% (full market rate) for US Bonds.
And that will be the trigger for an accelerated Debt Deflation.
Large swaths of the economy will lose access to credit, slowing growth. The combination of flat or declining growth, crashing consumer demand and high rates will drive a cascade of BK throughout the economy...small business, individuals, large corps, municipalities, states, banks. Everyone.
Except those who are holding cash.