I was an Xfce core maintainer from around 2004 to 2009. In addition to my work on Xfce, I also built a media player, and played around with some other projects that never really went anywhere. GTK2 had its warts, but it was generally fairly easy to build things, and it was fairly easy to work around things when something didn't work the way I wanted.

GTK3 changed a lot of this. Many things that were previously public were now considered private implementation details. Much of that was perfectly understandable (necessary, even), but occasionally this meant that there were things you could do in GTK2, but just couldn't in GTK3 (or could, but doing so was a huge pain in the ass). GTK3 deprecated or outright removed some things in GTK2, and it wasn't often clear what the supported replacement was supposed to be. The theming/styling system was completely replaced, which meant redoing themes, and any app that used theming APIs had to have that part completely rewritten.

On occasion I would report issues to the GTK folks, or would watch issues others had reported that I was interested in, and responses were all over the map, from helpful and welcoming, to actively hostile. What became very clear was that the GTK developers, over time, started only caring about the GNOME use cases. If you were using GTK for something non-GNOME, the GTK developers did not care about you, and would not reverse decisions that (incidentally or otherwise) broke your ability to do what you wanted to do.

I get it: as an open source developer, you have your own priorities, and you're usually not beholden to your users for features or any particular work. I certainly did not care for it when people not contributing to my software would demand I do things for them. But man, did it suck to constantly feel like the rug was getting pulled out from under me.

I haven't touched GTK4 yet, but judging from what I've read about it, I really don't want to. I used Qt a bit around the early '00s, and kinda liked it, but didn't love it (granted, I'm sure things are unrecognizably different now, for better or worse). Regardless, these days I have zero interest in writing C++. (To be perfectly honest, I have little interest in writing C either.) My hope is that one of the pure-Rust UI toolkits will take off, but at the same time that doesn't help existing C/C++ projects, since new-language rewrites are painful and often counter-productive. And on top of that, it just sucks to have more UI toolkit proliferation, which makes it harder to style things so the various apps on your desktop look reasonably consistent with each other.

Of course, my happy place would be Xfce rewritten to use a modern UI toolkit that is not GTK (or Qt, for that matter). (I would prefer Rust, but see above re: new-language rewrites.) But if we all thought the GTK2->GTK3 effort for Xfce took a really long time, rewriting using a new toolkit (let alone a new language) would likely take a decade. We were always understaffed when I was involved, with Xfce being a spare-time, volunteer, passion project for everyone involved (I believe one guy was paid for a while to work on Xfce-related things, but not for all that long). Not sure how much that's changed, but I wouldn't expect much corporate backing, despite Olivier (the project lead) working for Red Hat.