One of the biggest problems with GIMP and Inkscape is how GTK renders numeric inputs. Look at the size of those +/- icons on the toolbar. For drawing software with lots of numeric inputs, that means an insane waste of space. Proprietary software like DaVinci and even Photopea have much smaller inputs, allowing them to create far more complex GUIs and featuring without cluttering the window.

Tbh, you could summarise a lot of the issues with their UI as "GTK"

GIMP is far from a nice example of what GTK can do…

For sure, which is ironic given it was the application GTK was created to support. But even so, I don't think I've ever really seen a GTK-based application I actually liked (the rest of the UI is going to have to make up for the atrocious file picking dialog, for example)

A lot of these UIs were built around icons, following the direction MS Office took in the early 2000s. These once had colours and were somewhat distinguishable.

When they tried to jump on newer trends they made them all monochrome, and they don't have distinctive sillouettes so it's just completely inscrutable. That Inkscape screenshot is horrible. I count 129 icons.

And yet they still devote so much space to their bizarre colour palette, which is huge but still manages to omit most of the gamut? It's the kind of design that really only comes from decades of OSS development.