Scandium - you keep going back to the median student loan balance. That's what you are missing... the people who are at or below the median loan balance are not the source of the risk. It's the ~14MM people who have far more than the median balance we are worried about.
Same was true with the mortgage-bubble; the people who caused the problem weren't your traditional borrower who put 20% down and held a mortgage that was within 6x their annual salary. It was the huge number of bad loans (repackaged) which the home-owners had no hope of repaying once a relatively minor road-bump like a recession came along.
In that regard, it's the same thing. We've got tens of millions of people who have loans they can't get rid of which are draining >40% of their salary each month and they're one job-loss from defaulting.
Unlike car loans, there's no simple way of discharging a SL through bankruptcy, and there is no car for the banks to repossess.
But loan balance would scale with income, otherwise it makes no sense. Nobody would take out 2x the loan, unless the education you get from that give an equivalently higher starting salary. Maybe not 2x, but at least quite a bit higher. Say you can get an engineering degree for $50k with a salary of $60k. Then nobody would pay the same to get an education that only pay you $20k out of school (something you could get for less than 1/2 that). Education is an investment; you wouldn't pay twice as much for a stock unless the potential gains were much higher. See Amazon vs an old utility.. So the loan/income ratio should follow a curve (i.e. doctor with $200k loan and $250k salary). If anything I would assume the more expensive degrees are more prestigious, and more in demand. Those are the ones who should have little problem getting a well-paying job.
(In some cases there could be other reasons of course. For example a friend took longer to get his degree due to medical issues, but that's a tiny percentage)
