This is what forks are for, isn't it? If it's that simple, I'd think a fork could pretty easily start to solve some of GTK+'s issues.
From my perspective, GTK has been mired for a very very long time. Essentially from its inception it was something that was required for other missions. There have been tons of volunteers and people working on it but I'm not sure how much of that was because they needed it vs because that's what they love to do. I also have some suspicions that certain UXy/visual type problems just don't seem to be good fits for the bazaar model, all my evidence would be tied to GNOME, GTK, and GIMP for the most part so I could see how I might be wrong. If it's being held back by some Redhat folks that are paid to maintain it because Redhat went all in with it decades back, a fork could potentially revitalize it.