ColorStudio came out about the same time as Photoshop. The two competed for awhile. There was for a short time, a similar set of camps for which was superior as we saw with “Nikon vs. Canon”. ColorStudio had CMYK before Photoshop which was one reason some settled on it instead of PS. Ultimately PS became the victor, probably because it was IMHO a bit easier to learn and use. In early 1990 I had the choice and fortunately picked PS and never looked back. There have been products that were said to be Photoshop killers (Xrez, Live Picture), but they died too.
Well i (still )like the program because it is very flexible and starts at the basics ( channels, RGB)
That is why a lot of people can use it for very different needs.
That said there has not been much progress in some areas.
It makes hardly use of the multicore processors even when it is very easy to implement. (for instance at simple batch actions)
Sometimes it uses only one core while i have 10, so 10x slower than needed.
The GPU assistance lead often to problems. ( not well implemented, very old problem -already for many years giving)
Personally i have a problem with the Wacom, painting in masks, layers etc sometimes does not work well. Maybe they adressed it with this release - not sure yet)
( They had a plugin for it but that did not help 100%- it could be an Apple related problem)