A few years ago oracle DBAs used to (and some still so) dismiss MySQL and PostgreSQL saying they were toys that didn't scale. 10-15 years ago 'real men used Solaris' and Linux was a toy for hackers.
Any time any with any technology those that have mastered the 'premier' product, especially those that are paid because of their knowledge of it, will always think it superior.
That's why the second point is so important. It isn't just Adobe people (the oracle in your comment) or Silicon Valley startups dismissing Gimp, it is the in-the-trenches graphic designers that do their work for $12/hr in a small rural town. That's a huge barrier to overcome. It isn't impossible, but with your other examples progress in numbers was seen all the time with MySQL. Not so much with Gimp, right?
(Also I'm not like all-in for Adobe. I'd love to see something hit the PS market, but I just don't see Gimp being the one to do it. Something like http://www.pixelmator.com/ has a better shot, because they seem to be making good traction in a fraction of the time as Gimp)
Switching between one version of *SQL and another requires intellectual knowledge, while stuff like Photoshop involves a lot of "muscle memory". Remembering (slight) differences in syntax is a different type of cognitive task than remembering where to click to find everything in a mondo-complex UI like that of Photoshop or The GIMP.
Any time any with any technology those that have mastered the 'premier' product, especially those that are paid because of their knowledge of it, will always think it superior.