And yes, the designers aren't some godlike artisan savants that sculpt masterpieces 5 times a day, they are normal people, people that make make mistakes like going "live" with an experimental UI in OSs they knew where going to have to package it by default or switch to KDE and QT, which isn't an option either since that is also a bad move seeing how buggy and resource hungry it is.
Go take a look at every MS OS from Windows 95 through 7, the same UI is there either as the default, available in the display options or just under the surface if you disable the "Themes" service. Mac OS? No major changes from System 6 through OS 9.2.2, going into OS X slight changes but no massive departure from the paradigm.
So again, why should this kind of complete change be acceptable in OSS software? And don't say use "Fallback Mode," they are already going to axe it as we knew they would.
And before you ask again, yes, I've already tried and didn't like the way XFCE or LXDE worked. Mate is now the only good option but may not have the manpower to keep the thing going.