The big feature of Photoshop is that it's Photoshop. A typical designer has dozens of expensive plugins for Photoshop, their PSDs only work in Photoshop and they only know how to use Photoshop.
Changing would be expensive and risky, and would only be worthwhile if lots of their colleagues and clients were also moving.
So Adobe has intentionally created a lot of barriers to exit, and even if an amazing good open-source package came out today, it would struggle to gain traction.
There is one contender that's been getting some traction with a significant minority iOS designers; Sketch - http://bohemiancoding.com/sketch/
It's nowhere near an entire replacement but it has a very different take on the UI which arguably makes it better suited for app UI design and it appeals to the "cool kids" ... those starting out their design careers with app related projects as best demonstrated by this book / tutorial - https://designcode.io/
And that's probably the only way to unseat Photoshop - with a generational change.
There is also the printer-oriented color management in Photoshop, whereas Gimp appears more screen-oriented. Designs made in sRGB may look radically different when converted to CMYK.
If you go to print, the fact that Adobe's tools understand multiple color models is a HUGE advantage over most other tools, which usually only work in RGB. It's not just open-source competitors, it's smaller teams trying to compete in this space as well.
>>> Changing would be expensive and risky, and would only be worthwhile if lots of their colleagues and clients were also moving.
This is a huge point. Adobe is so entrenched in so many corporations, it's almost like MS Office to a degree. Even in most startups, if you want to hire someone, you're going to go with Adobe products instead of having a training/learning curve to worry about.
>> So Adobe has intentionally created a lot of barriers to exit.
Of course they did. Once you have an entrenched software platform, you want people to continue using your product, regardless of whether they like it or not. It's just smart business. Not really good business, but smart nonetheless.
Maybe that's true when dealing with clients who also use photoshop, but the majority of artists I know love to experiment with new software - and hardware - and flit between them depending on the job at hand. Photoshop is a bloated, expensive piece of software, but Adobe listen hard to the people using it, and it's really good at what it does, and that's why people keep going back.
Changing would be expensive and risky, and would only be worthwhile if lots of their colleagues and clients were also moving.
So Adobe has intentionally created a lot of barriers to exit, and even if an amazing good open-source package came out today, it would struggle to gain traction.