Wii is a fad in the same sense as every game console before it was a fad. These machines are built to only last for a limited number of years before technology and consumers move on. Traditionally the life cycle has been five years. The only difference between the wii and the HD twins is that they were developed with different life cycles in mind. The Wii was developed with the traditional five year life cycle in mind while both microsoft and sony developed consoles that could push past that limit by emphasizing console adaptability. The Xbox 360 and Playstation 3 both have incorporated (or discarded) a ton of features through system updates and redesigns while the Wii is essentially the system you bought from day one.
Basically, in the grand scheme of things, all game consoles are fads.