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.
And history was also showing us that Nintendo home consoles sales decreased with every generation... until the Wii came along and broke that trend.
History can be useful for showing correlation, but you have to examine it more deeply before you can make any conclusions.
To lavish praise upon this title, the assumption of a common plateau between player and game must be made. I won't open my unworthy mouth.