hence Wayland. No one beyond the Wayland devs has any say in what happens, limited politics, no guarentee of backwards compatibility and therefore no mandatory legacy cruft. that being said Wayland IS BC due to XWayland which is good.
oh also X12 would probably be forced to speak the traditional X protocol which you can see why on the Wayland faq that's an issue. also it was a chance to experiment with the idea of "clients in control" instead of "centrallized server in control" though Mir is going back on that.