Photoshop is not the best example because, at the moment, Sketch is eating its lunch and really cutting into its market share among web / mobile designers.
I think this is because Photoshop was never a very good UI design tool. Using Photoshop for UI work always felt like a hack. (Its text rendering and vector support have always been weak, for example.)
We all used it because there was nothing else out there for a long time, but now finally there are new tools more focused around UI design. So it makes sense that people would start migrating over.
But still, you can’t use Sketch to edit photos or any of the 1,000 other things Photoshop is really good at. Photoshop’s not going anywhere.
Maybe eventually Adobe will break it apart into a collection of more focused apps — seems like they’re starting to go that way. I have my doubts though! :)