The extra irony here is that both apps built their own, entirely custom UIs rather than settle for dependencies on existing kits... The big difference with GIMP

The extra irony here is that both apps built their own, entirely custom UIs rather than settle for dependencies on existing kits... The big difference with GIMP was that its toolkit was built to be separable from the rest of the app and eventually became its own project, while Blender's is just ... Blender. I suspect if you described the architecture of the two apps without naming them, most devs would predict disaster for the latter. But as you say, it has not worked out this way; indeed, GTK has repeatedly dropped or... gimped... functionality that originally existed to facilitate productive interaction with the GIMP UI (such as fast custom shortcut keys), while Blender's UI remains bespoke and quirky, but also insanely powerful.