History is back around to repeating itself. I'm an avid Linux user (and even a developer who uses GTK pretty regularly), but the recent developments surrounding GNOME, Flatpak and their related libraries has not made me optimistic for the future of Linux applications.
GTK4 is really bad. It launched with horrible text rendering issues (still unfixed, 10+ months later), broke compatibility with a number of basic features, and then had those features replaced by the GNOME middleware known as libadwaita. libadwaita is in a "functional" release state, but still lacks support for custom stylesheets, basic desktop integration on non-GNOME desktops and has it's own litany of bugs that accompany it. Fair enough for brand-new software, but the state of GTK4 is unacceptable and borderline impossible to use on a daily basis. If you're a developer, things only get worse. Gone are the days of fast app development with gtk::Builder workflows, and now Glade has been thrown by the wayside with no replacement. Talk about a second-class developer experience. And then there's the strange omissions that the GNOME team insists on; some issues are marked as WONTFIX since they can be mitigated with Flatpak. Some features (like appindicators) have been ignored completely because they don't align with the GNOME desktop's vision. The general demeanor of the team has been my-way-or-the-highway, so making suggestions mostly gets you labelled as a troll in their gitlab and sees you forcibly removed if you don't give up and assimilate into their opinion.
Flatpak itself is becoming the new Wayland. With more than 600 open issues on Github and outstanding issues like random data deletion, portal malfunctioning, compositor glitches, security holes and more, it's less like the "one package manager to rule them all" and more like "snap but it doesn't work". It's slow, doubles the dependencies that you store on your system (!!!) and doesn't integrate with your desktop unless you go out of your way to install questionable third party hacks that forward your native stylesheet and XDG options. I'd honestly rather use AppImage if given the option.
The future of app development on Linux is bleaker than ever. Fragmentation is at an all-time-high, and the technology that was supposed to fix it only fanned the flames into an unsalvageable dumpster fire. If you are a Linux developer planning on shipping an app, please stay on GTK2/3 for the sake of your users. I outright refuse to run GTK4, libadwaita or Flatpak on any of my systems. Nothing I've seen recently changes my opinion on that.
Dead dove, do not eat.