The only way any such system makes any sense is if the state is getting production in-kind from those graduates. Have them earn the money by working in a state agency or providing pro-bono tutoring for under-served students. Giving something for nothing will rapidly invoke the law of unintended consequences.
Simply paying debts based on a income level will drive a behavior. Not in all, but in some. Why would I work for $50K a year if the state is going to help me out as long as I stay under that level? Once the state has paid my debts, I am free to earn whatever I want without being responsible for the debt I incurred. This type of a program punished people who are frugal and rewards people who are not. It punishes those who work hard to get a better job and rewards those who do not.
The loan programs themselves were supposed to solve the problem of eligible college student being able to pay for college by effectively letting them learn now and pay later. The increased income from the college degree was supposed to allow students to pay back the money with relative ease and we get a higher-earning, better educated tax base…but then reality hit. The influx of dollars available for tuition drove up the cost of tuition for everyone and the only real result was the bloat of colleges and higher debt for all.
This is the problem with Keynesian economics. It assumes things happen in a vacuum and does not adequately account for behavior changes in people when policies are implemented. Keynes was fatally short-sighted. He was not interested in stability of growth and long term solutions. If he were a repairman, he would use duct tape to fix your pipes rather than solve the underlying problem and repair the pipes correctly. Eventually, the problem gets worse, not better.