I have used Gimp regularly and still like it, but it is... different, and not in a good way.
The fiasco that is exporting / saving for example, clearly shows that core developers don't care about users much. Which is fine, but it is a shame. I have stopped recommending Gimp some time ago.
You realize it needs to be done, right? :/ You realize this attitude is likely what prevents it getting done?
I’m sure I’m not the first person to go through that experience, and; again - this needs to happen in the actual build, not a fork.
Stepping in with a replace-the-whole-UI idea is generally combative in nature and may engender combative responses. Rather than being surprised at the response, maybe think about whether what you did might cause it.
By odd analogy, it's like you're starting a career as a traffic engineer and immediately demanding the road surfaces get replaced, without demonstrating an understanding of why the road surface is the way it is or why projects to replace the road surface tend to fail.
(I'm not involved with the project, so my attitude doesn't matter and you don't need to worry about what I think.)
You realise that coming in new and wanting to immediately overhaul the UI/UX is not a sociable thing to do, right? In fact, people new to a project suggesting a huge overhaul is generally a red flag for their understanding of the project or their likelihood of being a good-faith contributor.
I do agree that the UI/UX of several tools needs an overhaul. Given the history of such efforts across many open source projects, it seems likely that a fork is the only way to pull that off. You'd need a talented UI (and fixup dev) team to make it worthwhile.