This info is old. Right now the goal is to deprecate X and move everybody to Wayland, except for legacy Motif apps and the like which can't be migrated. The major modern toolkits -- GTK and Qt -- will remove X11 support some time in the future.
XWayland is only a stopgap/transition tool. Maintainers want to maintain as few code paths as possible; ideally one per platform. For toolkits that will be Wayland, not X11, once Wayland reaches critical mass.