It just doesn't make sense. History has shown us that Nintendo consoles peak early in their life cycle. Gamecube had price + games and that didn't help it against the PS2. Now, I'm not saying Wii U is another Gamecube but if we rate Wii U on a scale of 1 to 10 where 1 equals performance similar to Gamecube and 10 is closer to Wii......I would rate the Wii U a 5. It won't fail like Gamecube, but it won't succeed like Wii either.
I completely agree with your conclusion, that the WiiU will sell somewhere between GC and Wii (well, thats a lot of free space between the two). But to the first (early peak): I think SNES had good sales over a long time.