You clearly don't understand LightDM at all!

It is a backend (without a GUI) and frontend (called a Greeter) which can be whatever the hell you like.

This logic -> visual separation should make it a lot easier to maintain. The Gnome version will use GTK and have all the normal Gnome stuff. No-one gets an unsatisfying experience.

Also - LightDM will have a plugin-able backend to making, so it's already thinking of Wayland.

Have you worked on GDM? I know Robert Ancell (the main LightDM guy) has, therefore he probably understands the problem pretty damn well.