You have literally no idea who I am or what I do.

I used GIMP to make a mock-up of a sign for a restaurant just yesterday. Is it going to be the tool I use for the final product? No, because that'll be in vector, but it's a lot easier to slap something together in than Inkscape or Krita.

'Killer apps' are meaningless in comparison to useful apps. I'm an artist who needs usable tools for her work. GIMP qualifies. Personally, I find it way easier and more intuitive to navigate than Krita, Inkscape, or any of Adobe's suite. It may not be for you, that's cool.

But what isn't cool is to pretend you know about other people's lives and what they need. Speak for yourself, you are perfectly capable of doing that. If you don't like GIMP's UI, that's great. If you think GIMP's UI is absolutely horrible for every user and nobody would ever use it for professional work.. you're literally just completely wrong.