@Aury88 The problem here is not whether or not doing such transition, but how.
Unity8 is a complex software (~110k lines of code) which has a huge number of dependencies, from the Ubuntu UI Toolkit to GLib, from LightDM to LibIndicator and AccountServices, and all the other components specifically written for U8: Scopes library, Ubuntu Download Manager, Ubuntu Keyboard, MediaScanner2, MediaHub, Thumbnailer, and so on...
Mir is just one piece of the puzzle. Before touching any single line of code, it's important to know every single dependency of Unity 8, and be sure that all the building infrastructure works properly (i.e. being able to successfully run all the test suites of these components).
What really concerns me is not moving from Mir to Wayland, but getting a proper estimation of the development costs.
I'm not sure that going forward with the Canonical idea of convergence ("One OS running on every device") is the best way to get things done.
I have the impression that having a single shell on different devices (phones, tablets and desktops) adds a lot of complexity to a single software, and Canonical probably got stuck on that.
I haven't seen yet any other project trying to do the same: Microsoft decided to develop two different OS's: only "Universal apps" and the UI Toolkit are shared between the two platforms; KDE is doing the same, developing Plasma Mobile and a convergent UI Toolkit called Kirigami; and ChromeOS is not Android, but it's able to run its applications.
Also, there's another fact I haven't seen any mention yet: moving Ubuntu Touch to Xenial would actually break any compatibility with the apps and scopes currently available on the Ubuntu/Open Store. Canonical had an hard time (since 2015) to figure out how to do such transition, and they never moved from Vivid (i.e. Ubuntu 15.04) as base for their Ubuntu Touch images.