I don't see why Wii Fit outdoing the lifetime sales of PS3 or 360 would be impossible. Very unlikely, but Nintendo games have been known to perform in unlikely ways this gen.
I think Nintendo are actively planning to sell 30-40M units of Wii Fit. I think it's likely 360 will finish with lifetime sales at the upper end of that range. It feels like MS are running out of cards to play, while PS3 is still gaining momentum and hasn't tapped some of its core franchises, and so could end up higher than 360.
Last gen, games like the Sims, GTA:VC and SA, GT3 and Pokemon R/S were only a stone's throw behind the sales of Gamecube. Pokemon RGB sold roughly the same as the N64. 360 only recently passed Nintendogs in sales; 4 different DS games are still ahead of PS3 in sales. Having a game above a console in sales isn't impossible, and Wii Fit feels like it should be one of the highest appeal games ever made.
I'm pretty sure a few days ago, the idea of Wii outselling PSP, 360, PS3 and PSP combined during GTAIV launch month would have been laughed at. And a few months ago, people were poo-pooing Reggie's prediction that Wii would pass 360's install base in NPD numbers by June, though now it looks like it could pass it in May. Basically, a lot of people aren't grasping just how powerful Nintendo's momentum is until it hits them across the face.
I'm not making a prediction. As I said, its very unlikely. But maybe the folks who admit this is at least possible really aren't so dumb...
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
