The entire point of a new version is that it has newer features/widgets. Why even bother using a new GTK version if you didn't care about using the new features/widgets? In that case you could still compile against the old version and expect it to work as it always has.
If what you're looking for is a way to roll out some new widgets one by one and transition to a new version slowly, I agree that's something that could be worked on. But it requires a bunch of additional work that's mostly orthogonal to just shipping a new toolkit and it may not make sense for it to happen upstream.
