Contrary to public perception, there's no robust investment market for student debt. US government ends up purchasing most of it through Treasury. In theory it's a great deal, as it gets to borrow at low Treasury rates and originate at higher rates.
I don't have a link to the original source, but "In September 2018, the U.S. Treasury Department revealed in its annual report that student loans account for 36.8% of all U.S. government assets." https://www.investopedia.com/articles/personal-finance/08121...
Edit: here's a more updated analysis of the Treasury disclosures, which puts the figure at 51.2% as of September 2019 https://www.advisorperspectives.com/dshort/commentaries/2019...
Devaluing those assets (which Treasury marks to market) would significantly dampen (if not destroy) US government's borrowing ability as well as make some government entitlement programs [at least temporarily] insolvent.