Rootfull was already an option
You could run Xwayland rootfull and fullscreen already on the existing compositors. I did this in crostini with ChromeOS's compositor, but it should work on sway, gnome and plasma. Then I ran my x11 windowmanger of choice, evilwm.
I'm pleased wayback is getting momentum but it's only useful if you want ultimate lightweight layers/processes running (if Wayland+typical DE is bloated and slow on anything built in the last 15 years I'll be surprised and disappointed), or if at some point gfx drivers become wayland only but you want to remain very old school and/or still had an unacceptably weak machine. I'd assume just using Xorg is going to be better for years.
Wayback implements the bare minimum to get Xwayland functioning, so if you wanted to run a wayland only app like waydroid you'd not be able to just flip to the main Wayland compositor for that.
Now if it had future plans to allow your x11 windowmanager to move/resize rootless Wayland apps that would be amazing, but I don't see any mention.