A person's ability to pay off a loan is not the true value of the investment. There are positive externalities to getting an education that boost society as a whole. Real wage stagnation, rising income inequality, and compound interest on loans would all serve to make this investment less rewarding for individuals while not decreasing the overall societal value of education.
There definitely are positive externalities to education. But I doubt they are even within an order of magnitude of the benefits of training and signalling which accrue to the individual.
