In case of default, discretionary items can be seize and sold to at least repay part of the debt (while decreasing quality of life).
2008 has shown what seizing houses leads to.
Now think that there is nothing to seize for student debt loans.
If you fail to get a good enough income using the degree you paid using debt, you are as hopeless as if you had used it all on hookers and cocaine. At least with discretionary items you have a big TV which holds some value.
What we need is to make college debt discharable through bankruptcy, same as any other debt. Then the only people who can borrow are those likely to be able to pay it of, which nicely solves the problem of unplayable college debt (while reducing the interest students of medical, science, law etc have to pay, since more money will be chasing fewer students), for profit colleges and useless degrees.
Does it mean some people won't be able to study their favourite subject? In general no[0], in college? Yes.
[0]: Remember the " you dropped 150 grand on a fuckin' education you could have got for a dollar fifty in late charges at the public library!"? Today it is probably 15 USD on Beeminder, and you won't have a degree, but that degree is worthless in the first place, so overall huge win.