Sonic is still a fairly well known name. Many of the people who buy Nintendo consoles don't shop in GameStop and they don't read reviews. They go, "oh, mom, Sonic! Look at the cover! This looks cool!" That, or they say, "this looks like something Timmy would enjoy, let's buy it for Christmas." New Nintendo owners are thus relatively insulted against hype and look at everything available when they go to buy a game, not just the newest stuff. That means lower Day One numbers but a much longer tail.
All of which is to say that Sonic might have decent numbers before it's all over.
Assuming the Wii U also ends up with decent numbers.