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.