I think you greatly overestimate the hardware turnover in the Pi ecosystem. Pis that run and do whatever task they were set up for continue to work. New Pis (particularly Pi0 and Pi0W) that *can't* run 64-bit software continue to be deployed. The CM1 is slated to be produced for another 3 years, and that, too, is unable to run 64-bit software.
The "show stoppers" of the A-series (with the release of the Pi3A+) and CM-series (with the CM3 and CM3+) mean that there are 64-bit *options* where those boards are used.
I have gone on at length, previously, about the Pi0/Pi0W being a major reason why Raspbian can't go 64-bit (at least exclusively) and I haven't seen anything to suggest that is going to change in the foreseeable future. Or--perhaps--a 64-bit capable Pi0/Pi0W is that "secret project" that was mentioned in passing in the recent BBC article.