I agree that gtk2 is going nowhere fast, and we (as the GTK-users community) should be looking to move towards an actively maintained codebase. Gnumeric is a good example of why we need to. I just don't see a suitable desktop environment we could use today in .next that runs on gtk3 - although by the time .next comes out things may well have changed.
And to correct what you are saying, few desktop environment run GNOME3 (and NOT gtk3) but all major distributions have moved to gtk3 a while ago already. Ubuntu, SUSE, etc...
I'm not sure what you're getting at here, but it's probably just a matter of terminology. As I understood it, GNOME3 is a desktop environment which runs atop gtk3. Distros like SUSE come with multiple desktop environments, so as the tech stands, probably come with either gtk2 or gtk3 depending on which desktop environment you choose to install. Distros like Ubuntu on the other hand make completely separate (though dependent) distros called spins to let you select your preferred desktop environment, and I'd certainly assume Xubuntu remains using gtk2 for the time being.