I remember when everyone thought that Pokemon were just a fad. They are not movie/tv/toy driving force they use to be but Nintendo sure knows how to extend the life of a 'fad' until it becomes something more than a passing fancy.
Same with NintenDogs, Brain Age, WiiFit... Things gamers figured would never live for a long time are selling incredible amounts week in and week out.
So if Wii is a 'fad' then in Nintendo's hands it should last 5 - 10 years, which is all it needs to do to be an extremely successful console.