Comment by MBCook

You don't need feature parity, because there are many features used only by a small handful of people.

When the first iPod came out, it didn't have feature parity. No wireless, no FM radio, smaller drive, no Windows compatibility. But it was better at the important tasks (finding music on your library, controlling playback). Similarly the first iPhone OS was missing quite a few features compared to BB/WinMo.

If Wayland or Mir can do a better job for the 80% or 90% case, they could easily displace X11. The fact that they are cleaner codebases (since they aren't 20+ years of layers) means they can probably respond to new use cases or important gaps faster than X11.