They dropped Sonic as their mascot before. Remember the Saturn? There was a brief stint where 'Bug!' was their mascot.
Of course, that's part of the reason why the Saturn was a massive failure(along with inferior 3D graphics and a $400 price tag).
Sonic games still sell very well today. Without them, Sega wouldn't sell very much of anything. Sonic games aren't as important for the overall gaming industry as they were in the 16-bit days. But for their parent company Sega, they are even MORE imporant today(though with some of their recent output, including the game which this thread is about, you'd think otherwise). And that is why you'll continue to see a steady stream of Sonic output for many years to come. As long as people continue to buy the games, Sega will continue to make them. The same goes for Mario, Tomb Raider, Madden, Tony Hawk, Pokemon, or any other successful franchise.