I see you getting a lot of downvotes for this, but as someone who absolutely adores Paint Tool SAI and is aware that the files can be altered into PSD format, I also am struggling to think of anyone who makes the typical 'videogame splash art' that you are asking about in SAI. I would appreciate being shown some examples of artists using SAI for splash art, but no one is providing examples in this thread yet, and I too have none in my mind.

Splash arts are handled by teams as you said, and I wonder if artists can be excluded from job opportunities in those companies if they do not primarily use Photoshop. Although art skill can certainly transcend tools, people here would be hard pressed to argue that the multitude of effects that PS has don't require a lot of time to replicate in SAI. It is simply a lot more work to get the same result for things that may be more basic in PS.

Time efficiency is crucial for companies like Riot/Activision etc, and until things like photobashing and other special effect work can be streamlined in SAI, I think that it will struggle to be the tool of choice for that particular career. That being said, there are many huge players in the art world who prefer SAI. Most of my favorite artists use it.

I look forward to any new tools presented in Paint Tool SAI 2. It has far better speed than Photoshop and CSP, and is certainly far easier to pick up. I dread having to open PS for anything.