Romain Grosjean might be able to beat Lewis Hamilton in a race on foot, but that's not how the sport is played. Motor racing isn't about getting to the finish line by any means necessary, it's about driving, which is a skill - one that Sonic must learn if he's to succeed under these rules.
As for characters from other Sega franchises, part of me suspects they're being held back for DLC, by which time they'll be a novelty - much like the tertiary characters being added to Sonic Mania, which was originally sold on the merit of being pared back to just the three most popular heroes with all their annoying chums left out. Now it's suddenly a bonus to be able to play as Mighty the Armadillo and Dingus the Cucumber.