You could do that, I suppose, but the ability to pay back the loan and the downstream income potential of the graduate are likely to be very strongly correlated

You could do that, I suppose, but the ability to pay back the loan and the downstream income potential of the graduate are likely to be very strongly correlated to the student's financial background (really, their family's financial background). If you're wealthy enough, it basically doesn't matter what you major in while in college. But if you don't have a rich family, then it matters a hell of a lot. Students seem to already know this; at least when I was in school last, there weren't a lot of first-generation college students or immigrants in the liberal arts programs; the kids who were betting everything on their education were in the business or engineering schools, mostly. (Or sometimes preprofessional programs, pre-med was a big one.)