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).