> X is so old that I can believe it has too much cruft to clean up.
But there is exactly where it still makes sense, it is used in SO many places that until recently there was really no alternative. So cleaning up still makes sense, and where necessary that has been done over years of hard work.
But there comes a time where it might just not do anymore and some radical new idea is needed, so you get something like Wayland. If you would have redesigned X to be what Wayland is now there just would be no reason to keep calling it X (except for PR purposes, see Atari or Commodore).
> but it looks rather like just an implementation detail from the outside
So you could say the same for X, Windows and Mac Quartz, they all seemingly handle showing and moving windows, the rest is just implementation details.
I just don't get people's urge for "making one system", what would that gain you? Better yet, examples abound where diversity results in faster progress and better results.
Now, I see much more evidence that the people starting on Mir should have talked with the Wayland developers before running of to make something of their own. But time will tell I guess.