Writing someone a check in exchange for their future earnings "from now on" is a terrible deal. You're essentially offering a loan that can never be paid back, yet requires payments forever.
Loans are a good deal because the terms are known ahead of time, payments end when the principle is paid off, and the cost can be calculated.
The idea behind ISAs is that they try to align the interests of the student and the educator, as the educator only gets paid proportionally to the students' future earnings. ISAs also have limits on how long they can collect from the student and thresholds that have to be met for collection criteria, as well as an upper limit on payments.
What you're describing is just facetious and condescending to those with six-figure medical debt.