Well it's possible but you could also make the "Brain Age" argument. It sold right along with its sequel and was undeterred by it still even now. But I guess the main difference is Wii Fit Plus is Wii Fit with extra stuff
But I would still think MK Wii still has more selling power to begin with just because it can literally reach any kind of gamer. Wii fit might close itself off to a larger crowd (long time gamers, people who see it as something for what they aren't, etc) but Mario Kart is a game that is loved for its simplicity and fun and competed on regularly for its intensity. That is a combination for big sales because it is appeals to everyone.
So while I don't see Wii Fit fading too much (in the normal sense with a sequel) Mario Kart Wii will stay strong the entire selling cycle of the Wii... no doubt. So it could happen. I really do wonder what this bodes for Mario kart as a franchise after this. Mario Kart was already huge on SNES, N64, GBA, and GC but it just exploded in sales on Wii and DS. Next editions could just be colossal.
