There would seem to be two possibilities. Either millions of students are simply lazy, irresponsible grifters who just took pointless degrees for the lulz and refuse to work hard enough to pay off their debts, which they could easily otherwise do, or your general case isn't as general as you assume, given externalities.

Regardless, we're talking about trillions of dollars in outstanding debt. That's going to be a drag on the economy, low and middle income families are going to suffer, it's going to affect spending, investment and the likelihood of future generations seeking an education at all. I'm just suggesting maybe the problem isn't entirely the students here.

When did I say the debtors were the problem? I'm all for free education. A third option is that students have been propagandized by parents and society that "any degree will get you a job," discounting that when their parents went to college, it was cheap and loans weren't necessary (due to the government guaranteeing all student loans and disallowing discharging in bankruptcy thereby inflating college prices and thus loan amounts), and that there was less competition from other people getting degrees such that companies did hire people with any degree and trained them on the job.