But the academics can't "wait +/- 20 years" --- they have careers to build now. Grad students, post-docs, and junior faculty have to build a portfolio of work that will be impressive across many institutions where they might get tenure, or just their next gig. (Most would love to get promoted at MIT, but the odds are against that.)

If most of those institutions are looking at the same set of prestigious, closed journals, avoiding those journals is not an option for an ambitious academic --- and if MIT starts insisting they must, good talent will go elsewhere.

That's why this is hard.