Is there any consideration in having a separate build that is not a read-only filesystem, e.g. more of a standard Ubuntu install? If there were something like that, I would think it could stand as the main development branch that would allow faster upgrade to 20.04 / wayland / systemd / etc. Then it could be a testing ground that new releases of the "stable" or "consumer" read-only system would come from.
It would also not frustrate any potential developers who want "standard Linux on a phone" (mobian, manjaro, fedora, etc.).
In summary, I understand a lot of the reasons behind the read-only system for stability, for security, for end users, etc. but I fear it keeps away possible developer interest that would benefit Ubuntu Touch in the long run if it were more "traditional Ubuntu" on a phone?
Based on the reactions to comments that I recently posted in the "The road(map) explained" thread, what you (and I) suggested doesn't seem likely, or at least not likely in the near future. However, Manjaro Lomiri may fit your needs.