Roughly 1/3 of all borrowers drop out, implying (I think?) that the majority of debt is held by people who complete degrees, a substantial minority of whom obtain graduate degrees.
If the student debt crisis is about rescuing people for whom college was of no benefit (for instance, because they attended a for-profit school and didn't obtain a bachelor's degree --- the overwhelming majority of low and middle SES quartile students at for-profit colleges do not), the intervention needed to solve that problem is radically different and less expensive than "forgiving all student debt".