I must be the only GIMP user that has never complained about the UX. But I have never used Photoshop so I’m not fighting muscle memory.
I think it might be muscle memory. I started using GIMP in 2005 or so before the single window mode, and my muscle memory is tailor-fit to it. It felt like an extension of myself.
With GIMP 3, there are a lot of improvements! But it also breaks my muscle memory a lot. GIMP 3 is objectively better, but I find myself opening 2.10 regularly.
I started with GIMP and still pull it up first before worrying about Photoshop. For me, the "fighting muscle memory" comes from "huh, GIMP does it like this..."
How do you wanna complain if you dont know anything else?