Expecting that a piece of software undertakes an ambitious UI overhaul (which is no small feat) without one or both of those resources shows, to me, a lack of understanding.
One is what you wrote - that it needs a product manager and tons of manhours just to do what's essentially a refactor. Not to mention that GIMP devs said that many UI changes would imply backend changes too, because of limitations. And they are working on those limitations, among other things. Second part of the point is that Adobe must have like 50 people work on Photoshop, and I guess even more when you consider the overlaps with their other products. Compare this to the active contributor number to GIMP.
The second point is that many people compare it to Photoshop like Photoshop is a gold standard of usability. It's just as a nightmare as GIMP if you're not used to it. So, it's not really GIMP's fault that people are used to something else.
The other factor is that Blender was always a tool by and for actual companies doing real commercial work and understands those needs. GIMP on the other started as a university project that got picked up the GNU project, and has always been more of a 'research' project.
We ask for rationalization of suggested changes. For some reason people think it's hostile.
As far as I know GIMP does not have a face for the press, does not organize conferences, has no one contacting businesses for funding. And it has an unfortunate joke name.