You can't default on a student loan (unless you die or something).
And if the problem comes at the scale this theory is suggesting, then like mortgage defaults, people may just ignore the collection notices until they get special accommodations.
And some mortgage loans, in some states, are full recourse -- they can go after other assets (including bank balances) and paychecks. That they often don't reinforces my original point: there are plenty of levers creditors theoretically have that can't practically be employed against mass defaults and poor/unemployed people.