Yeah, I was annoyed by the name change, but I got over it. I mean, VYSE is playable! SEGATA is rumored! They, and the Panzer Dragoon track and the story mode, make up for the title. Plus, SEGA will still be on the cover, just in the bottom corner.
I think they chose Sonic as the headliner because he sells, as you noted, and also because having Sonic as the first word means it'll be on shelves next to all the other Sonic titles. Helps sales. I recall they retitled Raiders of the Lost Ark to "Indiana Jones and the Raiders of the Lost Ark" just so that it could sit next to the other two on the video shelves and help sales/rentals.