>This will kill the GNOME desktop. It’s a dead end for anyone but GNOME developers. Everyone sees it. GTK4 is just the latest demonstration of their incompetence and disregard for other developers.
Please don't use this type of hyperbole, I've heard this type of thing often in various contexts, and while it may be fun to say, it's inaccurate and it's thoroughly unproductive in getting the actual issues solved. What you view as incompetence and disregard is more accurately described as "lack of time." IIRC there are only really 2-3 people working full time on GTK, and most of their efforts are focused on getting new releases out the door. It's not as many resources as it may seem from the outside and they don't have time to do everything that people want. They depend on volunteer contributors to handle the long tail of issues and edge cases that come up with older applications.
If you'd like to improve GTK2 and GTK1 applications, please find a way to contribute and help out! I can help to point you in a direction if you want. (I don't think it's really much of a big deal if older applications look weird, the main concern with them is usually that they don't break. From this angle, GTK1 and GTK2 apps are not being left behind, they work perfectly fine even on a modern system) I'll warn you though, it's not trivial to get a legacy app to look and act native on a modern platform. If you really want to do a good job at it, you have to manually redesign each of the apps individually and it gets time-consuming.
I personally think it would be great if XFCE/Mate/Budgie/Cinnamon/etc had the resources to pay more full time developers to contribute upstream and collaborate more with the GNOME people, but we are not there yet unfortunately.