[sprash]

> The majority of X clients that users care about are ordinary programs

This is simply not true. The only "odinary" programs according to your definition that I am running are a browser and a terminal. All other xclients I'm running go beyond that and can not work with Xwayland or XQuartz. (a quick ´grep "^[x,X]" .bash_history´ reveals xautolock, xbacklight, xbel, xcalib, xcape, xdpyinfo, xdotool, xkill, xmodmap, xrandr, xrdb, xsel, xset)

[E1Q6Y57O]

Replying again because you edited your comment.

xbacklight, xcalib, xcape, xmodmap, xrandr, xset: These are utilities specific to configuring the X server. They're needed on non-X11 window systems.

xbel: I don't know what this is.

xdotool: Some commands will still work. Other commands that require interaction from the window manager might not work, but they won't work on some X11 window managers either.

xrdb, xsel, xkill: These still work, although for obvious reasons they won't affect native Quartz or Wayland applications.

By the way, none of those are ordinary X applications. Notice how none of them actually display any windows or having graphical interactions with the user? You know, that thing that X11 is designed to do? Put windows on the screen?

[E1Q6Y57O]

Which X clients are these? You didn't name any so let's just look at some of the popular and recent flathub apps: https://flathub.org/

I see a lot of games, chat apps, text editors, photo apps, office apps. These all will work fine in XWayland and XQuartz. But also, it's relatively easy to get them running on Wayland natively.