I think Photoshop seems to have found itself snowballed into its unfortunate position of accommodating to everything - yes i'll agree to that. But it's been a progressional thing - Photoshop has been around for a very long time, and a lot has changed throughout that time. Much like a web designer now needing to know how to code - compared to web designers back in the day.

Thats where tools like Sketch come in - they do a set range of things, and they do it to the best it can. (I say it like this, cause theres some bugs that bother crap out of me ha!)

Also another thing is that Photoshop is just another cog (albeit an important one) in the massive machine called Adobe. They have many, MANY users, applications, systems and internal politics (I'm sure they do, like any massive company). But what I'm trying to say is - quick and BIG changes like extracting a set of functions and putting them into a separate app is never going to happen quickly. However theres hope as they seem to picking up the ball game and becoming a bit more agile and focused.

I think I probably would disagree. At a certain level, and in certain industries, there is a high degree of bleed between different kinds of work - especially for video production or complex image manipulation. When Adobe dismantles products into multiple apps, some functionality tends to be duplicated across them, which then introduces its own set of issues (i.e., look at Premiere, After Effects, and Speedgrade, and the ways in which all three of these products can be used, in different ways, for colour grading, or the ways in which certain functionality ends up moving between Premiere and AE in subsequent generations).

The idea that applications are necessarily faster and more stable if their codebase is smaller isn't necessarily the case, especially if the alternative is moving files continuously across multiple separate applications, requiring intermediary tools (like Dynamic Link) to facilitate those transfers. Perhaps Sketch is a better tool for your workflow, and i think that's fine, but I'd imagine many PS users move across different kinds of functionality, and would much prefer to deal with a single interface for this instead of managing multiple smaller applications.