> GTK4 has been in development for a bit and has improved a lot of the internals. One of the great upsides is that it can take more advantage of the GPU when rendering the UI.
So there was a clear need to rewrite parts to fix internals.
> The Adwaita theme has also been nicely carried over and looks very similar to the GTK3 counterpart. ... > When I want to make a gtk4 application for mobile I would need libhandy, but libhandy for gtk4 is not a thing. The "solution" is libadwaita. This provides the widgets I need but it comes with the downside of having some of the worst decisions in application theming.
So, there we have it: it's not "UI folks just changing stuff", its a ground-up rebuild of a UI-language.
Now, whether or not that ground-up should be flat, modern, or just copy the old, is another discussion.