The issue with any software of this complexity is tech debt. If anything, GIMP should be a big warning sign about what happens if you bet on e.g. a UI framework which moves on without you.
GTK's development and release schedule is pretty aggressive, especially with the even releases being stable and the odd releases being unstable (which no other library on the planet decided to do so). A lot of cross platform software decided to a refactor to QT because of that, and I'd argue that the problem lies with upstream GTK and its failed API stability promises along the way.
In open source projects, you cannot afford to bet on unstable UI frameworks because you don't have enough time to spend on refactors like this. If every second subminor GTK version breaks your code, it's just not possible to keep up with that.
I'm actually impressed that GIMP wasn't fully abandoned because the sheer amount of code that needed to be rewritten is massive.