I think the core devs should make reasonable attempts to notify owners in advance and then cut to the last major version.

At the same time notify everyone of a policy/commitment to support only X number of versions and shift the effort/resources to forward-thinking development - REST API, consuming services, static site generation.

All signs are reading that the headless CMS + static site generation paradigm will rise and displace the baked-in admin + frontend UI/UX model. I for one would be very happy if core devs efforts were focused on future tech not supporting abandoned sites and owners who have backed themselves in a version.