First, the problem isn't specific to the rootfs being *currently* writable. The problem exists if the rootfs has *ever* been marked writable (and modified).
Second, as discussed elswhere in mail, it would be better to force a full update in this case, *not* refuse to update the system. The Ubuntu Phone model is that users should receive updates automatically and seamlessly; while users can fiddle with their filesystem, we probably /don't/ want to leave those users islanded on the currently-installed image version forever, but instead propose a full update, possibly with appropriate caveats. This will clobber any (unsupported) local modifications but at least won't leave them with an insecure and unsupportable device.
First, the problem isn't specific to the rootfs being *currently* writable. The problem exists if the rootfs has *ever* been marked writable (and modified).
Second, as discussed elswhere in mail, it would be better to force a full update in this case, *not* refuse to update the system. The Ubuntu Phone model is that users should receive updates automatically and seamlessly; while users can fiddle with their filesystem, we probably /don't/ want to leave those users islanded on the currently-installed image version forever, but instead propose a full update, possibly with appropriate caveats. This will clobber any (unsupported) local modifications but at least won't leave them with an insecure and unsupportable device.