I really don't think Daigo's sick of fighting games. I remember just recently seeing a twitter pic of him and some other GG players at a restaurant, fresh off of playing GG. I wouldn't be surprised AT ALL to find out that he does indeed still play other fighting games or plays some of the classics in whatever spare time he has.
The problem is I think two fold.
One is the fact that he's not just sponsored, he's HEAVILY sponsored. Twitch, CY games, HyperX and there's probably more. With every single sponsorship, the more pressure you have put on you to stay competitive. That means that he HAS to put all his eggs in the Street Fighter basket regardless of how he feels about the game. He probably can't spend the time to get good at other fighting games simply because the sponsors don't pay him for other games, they pay him for his SF presence.
Second is the fact that Daigo is simply getting on in years. it's just a sad fact that there is going to come a time where Daigo won't be able to perform as consistently as he has in fighting games. The reason could be just age, it could be him having a family, it could be him starting up a business and focusing his time on that it could be anything. Which is why I think Daigo is doing the smartest thing he can in using his image to form a brand. Now he has a manga, an autobiography, a weekly twitch show, t-shirts and other stuff. He's basically making sure that even if his fighting game skills do deteriorate, the fighting game scene will still have a place for him and provide a viable source of income. I could also see Daigo starting up a FG-related business or something based on his brand, maybe something similar to what Alex Valle is doing in the form of WNFs. That way he can still help foster the scene and still have a reliable source of income.
Say what you want about Diego Umejuarez, the man thinks ahead.