The X11 server changes would involve isolating clients by default, keeping them unaware of windows they did not create themselves... but allowing a whitelist of trusted clients that can see everything.

This is inimical to the design principles of Wayland, whose security policy is to isolate all clients -- no exceptions, no escape.

The 1984 Mac was largely a novelty, but it was one that demonstrated possibilities. There are non-novelty applications that are precluded by Wayland. Something like EXWM could not be done. You cannot write a standalone virtual-desktop switcher, it must be integrated into the compositor. Et weary cetera. Wayland is very opinionated about the kinds of programs you can write for it, and if you want to go beyond that you must traverse the step function into "write your own compositor".