I think this essay is missing a discussion on the theories of what make university graduates better paid. The two main theories are: 1. Human capital. People learn stuff at university which makes them more valuable workers. 2. Credentialism. Universities select (or pass) people who were already going to be valuable. Now let's look at what we might mean by "valuable": 1. You can fool hiring managers into thinking you'…
3. Those who are more able in the workplace also have improved chances of completing higher levels of schooling thanks to their ability (or lack of disability).
I believe this one is validated by the fact that even as more and more people attain a university education, incomes have held stagnant.