In most fields, tenured professors coauthor their papers with their PhD students and/or early career postdocs. So what would happen is that they would be throwing those people, whose career depends on publishing in a hyper-competitive environment, under the bus. Apart from losing the funding with which one can renew contracts to students and postdocs, etc. As a tenured professor myself, I have thought about this often (1), but there's almost always someone more vulnerable that I would be harming with that decision.
Honestly, this kind of discussion comes up very often, but it is really difficult, if not impossible, to get out of this situation via bottom-up action. Top-down measures for governments are pretty much needed for any meaningful change.
(1) Actually, to be more precise, all my papers are already open-access, but many of them are in for-profit publishers that charge the author thousands of dollars to publish open-access, which in my view is also a parasitic model and a scourge for science like paying for subscription. The specific thing I've thought about is not publishing in any for-profit publisher at all, but anyway the shackles that keep us tied to publishers are the same in both cases (modulo whether you have funding to pay publishing charges or not).
I don't think anyone who's done research would argue that, but a lot of research isn't mirrored on arxiv/etc when it could be. So I see a lot of these as education campaigns for researchers themselves.
Many/Some(??) journals now allow to publish a pre-print to arxiv and alike and do not count that as previous publication... So you don't get the paper in the "journal"-format style, but the content is the same.
So you can still continue to publish in journal, while the research is also still freely available. Usually you are also allowed to share the published article via email, just not put it publicly available on your website...
I think the arxiv / pre-print workaround could have so much more potential if more researchers were motivated/educated on it. I'd love to see an effort to simply email the top 10k authors to request preprint copies to be hosted. If all they have to do is send a pdf, I think most would be happy to do it.
If you don't publish it in the right venue nobody will read it, which means nobody will cite it, which means you did the work for nothing. At least in all fields where I know people.