[nailer]

Being the defacto windowing system for a platform that has very little desktop market share isn't a great success.

[omail]

Being the defacto windowing system for the most popular operating system family is a great success.

And if you believe Windows is the most popular OS family, then being the defacto windowing system for the second most popular OS family is still a great success.

[nailer]

Actually the defacto windowing system for desktop Unix is OS X's window manager, based on Quartz Compositor.

[omail]

I wasn't referring to desktop systems exclusively, but if I were then you'd have been right.