I agree that's the theory, I disagree that it actually is what would happen in practice. E.g., if the government stops guaranteeing the loans I seriously doubt banks will accept "future earning potential" as collateral. So I disagree that it's actually collateral (your initial claim). The real collateral is the govt promise to back up the loan.
So the question still stands: if the government no longer guarantees the loans, what is the proposed solution to prevent banks from no longer lending to students?