Proposal to address the rising cost of higher education

Students in California have a proposal. Rather than charging tuition, they'd like public universities in California to take 5% of their salary for the first twenty years following graduation (for incomes between $30,000 and $200,000). Essentially, rather than taking on debt students would like to sell equity in their future earnings. This means students who make more money after graduation will subsidise lower-earning peers.
It is not clear if this will provide adequate revenue for the university. It also means the university bears more risk, because the tuition it will ultimately receive is uncertain. But the proposal will benefit some students and the principle is not so ridiculous. American universities already practice price discrimination based on parental income. The more money your parents have the larger your tuition bill; richer families already subsidise poorer ones. Why not price discriminate based on future income of the student rather than the current income of the parent?
It also means, in many cases, that degrees that command a higher value in the labour market, like engineering or computer science, will cost more than other degrees, like theatre arts. But if an engineering degree is worth more shouldn’t it cost more? If you think of a degree as an asset which pays dividends in future wages, the asset with a bigger expected pay-out should cost more. Faculty in high-value fields tend to get paid more. Perhaps some of that cost should be passed along to the students.
Incentives would also change; maybe university departments would become more invested in producing sucessful graduates. But might this undermine the mission of American universities, which is (or is often assumed to be) to provide a well-rounded liberal arts education? If universities become more income focused, will low-yielding, but socially valuable fields like philosophy wind up short of resources? To some degree, the university-for-all model already undermines our idyllic version of university. As more of the population goes to university, and must pay for it, more esoteric subjects naturally become less popular.
A trickier concern may be what happens if this approach is not implemented everywhere? If you know you will study engineering and earn a high salary wouldn’t you then opt for a school with a fixed, up-front cost—assuming that means you’ll come out ahead? Then would all the talented engineers go to other universities and potentially undermine California schools?