[CuriouslyC]

The key value of making STEM degrees free isn't that it will cause a massive increase in the number of people who get them. I suspect that the increase would be rather modest. Instead, it is to free people who pursue STEM degrees from the burden of debt, so they're more able to take the kinds of risks that can result in big wins for society. Innovation is an inherently risky pursuit, and if we make potential innovators risk-averse through debt we're wasting much of the value of their education.

To be fair, I'm not totally against (at least partially) funding certain liberal arts degrees either. Specifically, creative writing and journalism are both valuable. For example, it would be profitable to subsidize many thousands of creative writing students to produce more popular writers like J.K. Rowling or Stephen King. Total yearly sales of fiction books in the US is ~14 billion, taxes from that could cover subsidies for creative writing if they were merit based.