>Shouldn't there be a metric built in that says something like "the quality of education didn't meet the expectations of the loan, so the loan can be discharged"?

How do you prove whether the failings are because of the student or the school? The fact that there are three parties involved (the student, the school, the lender) also complicates things. Finally, lenders would bake this risk into the loan itself, which means higher interest rates for people going to non-famous institutions.

> Shouldn't there be a metric built in that says something like "the quality of education didn't meet the expectations of the loan, so the loan can be discharged"?

There are, in fact, several policies in this area, the most significant is Borrower Defense to Repayment: https://studentaid.gov/borrower-defense/