The fact is that the design of X is archaic and not well designed for the needs of the modern era. The benefit to end users is that rich multimedia software becomes much easier to develop for Linux so they get more of it.
I'd like to use keycodes > 255 please.
Wayland has crippled customizability compared to X11. This customization is much more important to the user because, as you said, the improvements of Wayland are not relevant to the user.
Bad sysadmin probably put xhost + in the main xinitrc.
Common rookie mistake, but don't blame X for that weakness.
It also wasn't anything malicious, just a prank, when you run xeyes on someone else's terminal. They would laugh a little and then continue with their lifes. Nothing serious, requiring sysadmining.
Oh and another application: Mapping a gamepad to keyboard and mouse for games that don't otherwise support gamepads. (See http://qjoypad.sourceforge.net/ and my fork with additional features https://github.com/panzi/qjoypad)
If I can't use something like QJoyPad under Wayland/MIR I won't use Wayland/MIR.
This is also a security problem. If I have the means to run a program (downloaded from the internet; don't fully trust) in a sandbox that restricts its filesystem access, it'd be nice to also not allow it to control my whole GUI session.
If it's perfectly fine, then you don't have to upgrade.