Treasuries are sold at auction.
If there are few bidders, most debt would go unsold.
The Republicans in the House might refuse to retroactively authorize Biden debt. And some of the leftist ‘rats may decide to screw the Biden debt buyers too. Some leftist ‘rats hate Biden and they generally hate financiers too.
If the Biden debt is sold at too steep a discount, it may be repudiated by Congress generally.
What’s more likely to happen the Republicans repudiating debt or the Democrats using the 14th Amendment to increase the debt, to me that’s easy the Democrats have no issue “trying things” while the Republicans doing nothing