For example, how do you prevent people from finishing college, immediately declaring bankruptcy, and then getting a high-paying job with their degree? Unlike a house or car, the education can't be repossessed so the holder retains all of the upside.
It would be the ultimate financial hack to get a medical degree, declare bankruptcy, and then immediately start practicing once the bankruptcy went through.
So obviously there would need to be a lot of protections against abuse. Perhaps a provision that the student loan could only be discharged after a protracted period (many years) of too-low income.
Ironically, there's a huge resistance to this idea from within humanities departments. They fear (rightly so) that such provisions would make it harder to get loans for humanities degrees because they pay significantly less. I don't think that's really a bad thing, but you'll find some outspoken arguments against this from people in the academic world.