RedHat pushes hardest, they basically do GNOME, GNOME does GTK. So GTK will be first --my prediction-- to drop X11 support.
People that prefer X11 is not what I'd optimize for. To me X11 is deprecated, unmaintain{ed,able}. It became too much of a stumbling block for Linux and other opensource OSes. It's painful, but we have to move on. Wayland is not perfect but it seems to be doing the job lately. In two year --my prediction-- it will be more stable than X11 in all aspects.