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.
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.
Sketch provides a clean, focused UI that makes it easy to get many tasks done simply/quickly. Specifically, icon design in Sketch is excellent.
Plus, Sketch is a vector based application which is excellent for creating multiple icons / images of different sizes.
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.