Between Mir, Wayland and X12 i prefer... X11 !
Now seriously...i doubt that anyone will ever make a X12.
Mir is distro specific....or at most will be used by direct derivatives.
However, we can not disregard the weight of those derivatives.
Way too many distros are Ubuntu derivative...will the responsible for
those derivatives capacity to force and maintain Wayland against their
main source for their distros ?
Mir also seem very imature...for now.
Wayland is waaaayyy too new...you can bet i won't want to be a guinea pig...i want something truly stable...Wayland at least for now won't work (if ever will) with blobs and for *now* blobs are way faster than OSS drivers.
*I* want max performance...when max performance can be achieved by Wayland (or Mir), i will have no problem using it.
Then there is the Enterprise sector very committed to X11...just like they are to sub-D connectors (AKA VGA connector) , PS2, etc.
(an example...at least in Europe...any LCD OEM continues to make and evolve 1280x1024 LCDs that, AFAIK, continue all use only sub-D connection...there is a reason for that...funny thing is that is also my favorite type of monitor...16:9 is fantastic...for a TV/Living Room but besides that i prefer good old 5:4 )
Most natural thing is Enterprises continue to use/want X11.
So , yeah, X11 will stay with us for a long long time...even at Desktop level.