Earlier quoted context omitted.
The Linux desktop will come when Microsoft loses its leverage over hardware manufacturers and they start to preinstall linux and stop putting up barriers to adoption (e.g. remove windows -> lose warranty). It's already better than the windows desktop (xubuntu anyway, not unity, however). The barriers to uptake are no longer technical and haven't been for some years. It's all market + legal now.
Remember the netbook craze? They put linux on there, but they didn't start to sell in volume until they replaced it with windows. When people buy something in a PC form factor they expect it to run windows. Linux on the desktop will not happen in a traditional desktop form factor because people don't want it and/or don't see the point. The one possible exception are chromebooks, because a laptop that only runs chrome…
Those were gimp versions (distributions) of Linux though. We haven't seen what happens with a properly supported straight-up Ubuntu or such full-featured desktop.