The return window should be time limited so that the uni shoulders some of the risk - that their degree has market value. Masters programs can be particularly egregious, they are profit centers where only a small fraction of those getting the degree advance to a PhD program or some position where the degree matters. If the time is limited and one only need to pay a % over a threshold pay, the uni has some skin in the game and can lose on the gamble. In this market design they may be more careful about the promises they make and better guide our population towards programs that have better market value (and less personal debt).

So there would be an incentive for a university to actually teach marketable skills.

Of cause it is all wishful thinking: big corrupt institutions exist not as if nobody can come up with better solutions