I think what we need is more developers...
GTK2 was great, GTK3 is better in many aspects, but they dropped things that not enough software used and that there were not enough people to maintain... GTK4 will be a continuation of that: better, but more specialized libraries for GNOME's use cases.
Forking GTK*, and/or forward-porting parts from "the good old days" sounds great in theory, but who's going to do it? Who's going to maintain it?
Off the top of my head, the MATE team is growing at a rate of, what, 1 developer a year? All volunteers too.
GNOME (and thus GTK) have hundreds of developers, many of which are paid to work on it. To expect them to stop and wait for us is unreasonable when we're the ones that chose this path.
I think what we're doing is fine: we backport what makes sense from GNOME, we add our own features every now and then, and we make wishlists to keep moving us forward. It works. We have an amazing desktop environment that makes us proud and happy. Some features will suffer with every upgrade, but we add other necessary ones too.
In the end, we do the best we can to keep things balanced between "traditional" and "modern". That's probably one of the hardest things to pull off, and yet that's where MATE really shines.