Qt

So yes, there is no perfect solution. Maybe KDE / free software developers who know well C++ and Qt could have a good chance to be hired, improving the situation (at least technically, most probably not for the licenses and legal matters).

I've developed with GTK during many years (versions 2 and 3), even contributed a little to GTK and GLib. Unfortunately they lack resources/developers, so the core devs decided to break some of the APIs (especially the low-level ones: GDK, the addition of GSK (scene graph), and how to implement widgets, and to leverage the GPU).

Thankfully most GTK-provided widgets still have mostly the same API. But still, I think it's hard to port to new major versions, especially a large codebase of course (and from my experience it's not linear with the project size; so perhaps a good idea is to implement the Adapter design pattern, providing at least internal compatibility layers, or if they are only related to GTK, provide them to the rest of the community to ease the 3 -> 4 transition).

I've heard the GTK drawing is better than the Qt counterpart. And with GTK 4 even more so, probably (but don't know well if it's still the case compared to the new Qt 6).