There are few Wayland only apps today. But as the percentage of Linux desktop users on Wayland goes over 80 percent, that will change.
And today, GUI toolkits and desktop environments have to limit features to those that will work in both environments. But not for long.
My guess is that it will be totally impractical to use an X11 only desktop 3 years from now. 5 for sure. In fact, I bet few people will even have Xwayland installed 5 years from now. To run what? Xfig? Netscape?
But certainly it will still be possible to run an X server. Xorg itself will still probably run fine. It already uses KMS and DRI from the kernel and those may not change much. And I am sure at least a few zealots will still be pushing XLibre a decade from now. And Wayback will almost certainly let you run an X11 desktop for many, many years to come for those “legacy use cases” you talk about. Like running CDE for a couple of hours.
Probably the most interesting development has been Phoenix. They have floated the idea of having an X11 first environment (eg. using an X11 window manager) that can run Wayland apps too. If that actually happens, I am sure it will find some fans. If you have spent 15 years fine-tuning your FVWM or Xmonad config, you won’t want to give it up.
It will be interesting to see where the BSD world goes with all of this. I think FreeBSD will go Wayland. But NetBSD and OpenBSD could be x11 holdouts. But the Wayland-only app problem will impact everyone. Time will tell.
Xorg fans will not accept that people like Wayland better because it is a better experience: higher performance and less jank. But that is the main reason and the reason that 80 percent of new Linux desktop users start on Wayland and will never switch. It does not matter if you believe it.
And of course the “killer feature” of Wayland is that it runs Wayland apps. And Wayland-only desktop environments and compositors. This will matter more and more every day. I could live without foot and COSMIC but already the fact that I cannot use Niri on Xorg is all I need to know to be Wayland exclusive.
But if you need an itemized list:
Waypipe and WPRS are better than the X11 equivalents.
Oh, and inevitability.