Here's the thing about student loan debt:
The vast majority of people who have unmanageable, unsustainable student loan debt do not go to public colleges or universities. They go to private colleges, where far more of the expenses are passed on to the student. So Sanders's free public college plan is not going to stop the most indebted cases from happening—unless he also cuts off all financial aid to students who don't attend public colleges.
In that case, the private and religious universities would crumble. Consider what's happened in Canada: of the dozens of major universities in Canada, all but one of them are government-operated (the lone exception being Trinity Western, whose tuition is comparable to American private colleges and which is under constant assault from the provinces over "religious freedom" rights).