X11 should be dustbinned just for the lack of multi-target network transparency. You know, the limitation that says while OSX and Windows users can efficiently share apps and desktops in a teleconference, Linux systems have to use VNC to toss around bitmap deltas instead. Its like getting a shot of Novocaine in the mouth everytime you head out to a party.
Oooooh, wait! Did I just attack X11 on its hallowed territory... Network transparency?! Well, indeed I have and its true that X11 has not gotten any overhauls to support this very important and common use case.
Security also stinks to high heaven on X11, and it took an OS like Qubes completely re-worked around a VM security model to address that architectural flaw (regular hypervisors like VMware won't even protect you). The priestly developers of X11 implementations do not appear to give a rats ass.
This stack (and its anachronistic neckbeard clique) has run its course and should have been on its way out 10 years ago. I think you're wrong about developing replacements for X11; Apple users never regretted it for an instant.
BTW, I don't know about you but I'm tired of my Linux UI's being interspersed with character-mode upchuck, screens flickering and popping momentarily in an out of existence whenever something different happens in the runlevel or login status or number of displays.