The important thing for Gnome is not the toolkit, but the applications.
That is the most important thing for any platform and is why KDE4 crapped out
early on. No apps means completely worthless.
So unless you want to have the Ubuntu desktop be a completely unusable for
the 3 years while people sort out the mess of dropping support for 90% of the
applications that people use on Ubuntu then it is probably a better idea to
just keep improving Gnome incrementally.