For the non-free content on JSTOR, I don't see much that another archive could do about it. The main issue is back issues of journals published between 1923 and

For the non-free content on JSTOR, I don't see much that another archive could do about it. The main issue is back issues of journals published between 1923 and now-ish. Future content could go open access directly online, in which case JSTOR doesn't even really enter into the picture. And JSTOR has already made all the older, public-domain content freely available. But in between, there is almost a century of copyrighted content controlled by the journals. JSTOR scans the paper journals, but can only make the PDFs available with agreement of the copyright holder. They could try to pressure them more, e.g. by threatening to evict journals from JSTOR if the journal doesn't agree to certain minimum licensing requirements. But they don't ultimately control that content themselves.