The idea of such changes is to remove old cruft, add new stuff where it was not possible before. As well to rearange stuff if it turned out to be far from ideal.
Same was true for KDE 4.0.
Yeah I'd like it if Gnome was written in Qt, yet that is just an illusion that won't come true. It would just take too much work for gains that would not justify that. Plain simple: Gnome works pretty well with GTK+ already so rather improve GTK+ than to start over.