> I think the GIMP hate is almost entirely down to the difference in its UI from Photoshop.

I disagree. I use Affinity Photo 2, which also has a different interface, and it's so much easier to use than GIMP despite having more features.

in earlier years, but in a way to this day, a significant factor against GIMP was not the UI - people could get used to the UI and did even when grumbling about it.

A big problem for a chunk of possible users was that GIMP's colour support sucked (and I believe it's still not really fixed). The moment you wanted to work outside RGBA at 8bits per channel, and maybe a bit playing with indexed color, you were in for a lesson in pain. And a lot of people wanted a tool they could earn money with, and that meant for a long time at least some amount of print work. And print, even digital printing, means CMYK. Later on photography started demanding HDR features. Even web these days will deal with non-RGB color spaces, and I am not talking about HDR.

Meanwhile GIMP's engine for years even if in UI you technically could select colours in CMYK, they were internally converted to RGB for calculations, then converted again. CinePaint, aka FilmGimp, started because people could not get patches for 48bit RGBA into mainline. And so on, and so on. Meanwhile Photoshop and other competitors would not only have a less divisive UI, but also additional features (I knew people who would choose Photoshop just for included pantone colour database).

Yeah and also Krita exists in the open source world and is very nice to use. People are just in denial about GIMP.