[haaaaaaryf]

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.

[akbar501]

Sketch is very nice. I use both Photoshop (when on Windows) and Gimp (when I'm on my Ubuntu laptop), so I have experience with each.

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.