There is roughly 1.6 trillion dollars in student loan debt owed. 40% ($640 billion) of that will be defaulted on. There won't be any large derivatives markets set up to eat it like there were with the mortgage lending crisis. The people who will end up footing the bill for this won't be insured banks, it'll be John Q Taxpayer, and the economy writ-large. Even limiting to $20k per person won't solve this issue.