The replacing of Gnome has already begun. They replaced Gnome Shell with Unity and GDM with LightDM. They're using Qt more often now as well (I believe Ubuntu One is Qt and I know that Unity2D is too), so it seems that perhaps GTK+ will be next.
This is becoming even more true as Steam will probably only support Ubuntu. Games are where the desktop users are. Gnome can spin its wheels and bask in its own self-righteous awesomeness but if the users aren't there, then it's all for nothing.
If they want to offer better binary compatibility between releases... well they can't, because they don't employ those guys.
If they want to support a new OpenGL release... well they can't, they don't employ those guys.
Ubuntu is crazy popular, but people really underestimate the extent to which they are an epiphenomenon.
Has Valve said that they will only be supporting Ubuntu, at the moment they are still in a private beta for the initial port to Linux, and if it goes well I suspect that we will see Valve support steam on non-Ubuntu systems. Even if they do not officially support other systems, I suspect we will see people make on-official ports.
RHEL 7 is based on Fedora 18, and will ship with Gnome 3.6 including the shell as default. It will be interesting to see how many corporate clients start using fallback on their desktop clients... and I saw a note somewhere that XFCE may be made available in the RHEL 7 repositories.
I say this as one currently using Gnome Ubuntu Remix on a couple of machines to see what 3.6 is like - its actually quite nice in my opinion.
Users of Gnome 3 are being very vocal with genuine (and some not-so-genuine) complaints about how the design process is going and what the final product is like. Since the Gnome guys are now in the cult of design dictatorship, they are always right no matter what--even at the expense of their own users--and their users are wrong to complain. The result will be an ultimately failed product. (I would bet money that at some point in the next 2-5 years Ubuntu will fork significant parts, if not the entirety, of Gnome--and when the 500lb gorilla picks up his toys and leaves, the game's over.)
Yes, a product must have some kind of vision, and at the end of the day someone's got to implement it, regardless of their talent. But humble designers recognize complaints and the needs of their users. Design dictators ignore them, because the dictators are by definition always right. That's a bad attitude to have, because most designers aren't perfect--but the human ego loves having control and loves being right. When people are told that success means being opinionated, it's a very easy thing to agree with, because everyone thinks their own opinions are the best.