Did you miss the GTK+ 3.0 drama?
The GNOME idiots have been making it a point to break compatibility and remove "old" (aka "working", "currently used") features. You are delusional if you think they will continue supporting X once they declare the Wayland version to be "standard".
Of course, they'll probably use their typical victim-blaming approach where claim that keeping the old version around is "too much work" that should be done by someone else.
I have heard of some compatibility issues between GTK+ versions. But my impression is that one area that GTK+ 3.x improved much was the multi-backend stuff. For example, the X11 backend is no longer mutually exclusive with other backends. And there nowadays seems to be a proper OSX backend which, in my opinion, gives a much nicer user experience than what you'd get with GTK+ 2 on X11 on OS X.
Of course it is possible that the X11 backend eventually gets abandoned when it is not deemed important enough to continue spending resources on its maintenance. But currently it is there and it even is likely the backend that gets the most attention, given how it is the default in most distros. I would not expect the X11 backend to suddenly disappear even after Wayland has possibly become the default display server in some GNU/Linuxes.