I think it might be possible to slop-code an Excel clone because — truly minor interactivity quibbles aside — it can be tested with a truly exhaustive suite. The clone can be proved to do what it is supposed to do, over whatever subset one would choose to implement.
But Photoshop is not an application in the same way that Excel is an application.
(Not least because while powerful, Excel, like Word, is an implementation of an existing idea. Photoshop is the OG; it has all the history of the OG)
Photoshop is also a surface and a medium. It has a feel, it has responsiveness. It's like oil painting or watercolour: its own thing, with its own pen/brush dynamics, its own interactivity.
Part of the reason people don't want to switch away to Affinity (even though Affinity is free as in beer, highly capable and in some ways better) is just that to a set of very hardcore users, it does not have this feel. They just don't like how it feels.
Automated testing that focusses on pixel-precise rendering misses the point. It may actually matter less that it renders PSDs pixel-accurately than that it actually feels right.
If you are not convinced by my argument, look at "dodge and burn retouching". For better or worse, high end fashion photo retouching is done in an extremely precise, wildly technical, tiny subset of Photoshop tools.
If you want to replace Photoshop it has to be responsive at that level. And since that level is partly subjective experience, it's going to take some people.