I agree that would be bad, I've tried to use apps with giant menus in the hamburger and it doesn't work well. However I think that highlights a underlying problem that is much worse: Giant menus cause problems in general, and GIMP has way too many menu items and way too many nested sub-menus. This will probably be a worse issue if GIMP ever ports to GTK4 since the "cascading" behavior from the old GTK1 menus was removed... And I think this already gets worse in GTK3 because the tear off menus are gone.

The designs are unfinished, but to me the idea was to move most of the functionality from the menubar into more tool options, or into smaller context-specific popup menus. I think if someone was to do a more extensive design review then that would be possible. But it would be a lot of work and changes to put on the users. And likely users would still want the menubar on MacOS where it's required. A good example of a really complex app I've seen that does it right is Blender. Siril is interesting for a GTK app, there is also GNOME Builder.