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?
People could be trying all sorts of things like flatpak / snap / different apps, etc. I know there is Libertine but there is some performance issues and it isn't available everywhere, etc. So take the chains off and see what happens? I suspect a large growth of the community (???)