My reading of the front matter is that, in essence, our present regime of student creates a problem of adverse selection. Adverse selection exists when two parties want to make a deal (sign a student loan), but one party knows much more about future performance than the other (the student). This makes transactions inherently very risky, as anyone who has taken private student debt understands as a fact of their interest rate. The US government bears the adverse selection risk in the interest of macroeconomic strength.

But what if private parties could accept equity? That's to say, a percentage of all future salaries, as opposed to debt, which is a fixed amount per year on some schedule that produces interest income for the payee. The authors of this paper primarily found that private information — i.e. I know that I can earn a good salary with or without college, or conversely, I fear that I cannot — drives "Willingness to Accept" loans and thus prevents private markets that fund college tuition from taking hold. They appear also to support an expansion of college equity positions on the part of the government, although I did not dig deeply into that position.

This is not an authoritative synopsis, I am merely an MBA with a long history in tech and an abiding interest in economic policy.