Sonic lost his power when Sega went 3rd party. Fans of Genesis loved Sonic because he was 'their' Mario, the cooler alternative.
That said, he's still a decent brand. There are lots of old fans of Sonic who would buy a system to play his games, and there are young fans who see the cartoons on Saturdays and want some Sonic. Sonic:Lost World will move some systems, and be a relevant factor for many when deciding what system to buy. I don't think it will push millions, but it will be relevant, especially with other big titles hitting Wii U.