The issue is that people learn Photoshop, they don't learn the fundamentals for the tool.
I'm not sure how that would effect sales. Are people who learned Photoshop without understanding how it works really be likely Linux users? I think the overlap on those sets is vanishingly small.
So they switch to GIMP and then find it's horrible because their skills don't transfer and they cry on the internet that "GIMP SUX" because they don't want to relearn anything.
First of all, I can tell you that I have used a significant number of Photoshop tutorials in GIMP to do various functions and found that they work just fine.
Second, the most critical (by frequency of use) tools in Photoshop are the technical adjustments - color, levels, curves, etc. They work the same in GIMP and are even in the same menus. There is no significant relearning to do. My wife uses Photoshop and Illustrator (as well as InDesign) professionally on a daily basis. A while back we were traveling with only my laptop, which has GIMP and Inkscape but nothing from Adobe. She was able to get by just fine for a quick job while we were out; going well beyond the use level that I get from GIMP even though I use it almost daily.
This is even worse in a business situation because relearning things pushes back deadlines and impacts quality
I'm not sure how this applies. How many businesses are running Linux workstations and need Adobe on them? Again this seems to me like a likely very small set. I don't see the absence of Adobe software in Linux as being a critical impediment to Linux migration for businesses who want to do that, either.