Fact is simply the wrong word; opinion is the word you are looking for.
I say it's a fad because I believe it is a short lived craze, a craze that will only last for the lifespan of the wii (another few years at most). The fact that wii game sales are considerably lower then other consoles shows that the audience is very different. Now let's look at kinect sales which are certainlyindicative of a fad: http://www.computerandvideogames.com/327350/kinect-sales-pattern-unhealthy-ubisoft/
Motion controls, right now, are a fad for gamers. The fact they have such high latency proves they are not made for gamers but impulsive shoppers and casual gamers.
We're talking about a game console here. Selling well for a couple years and normally for a game console already puts it beyond fad status, there's no way you can possibly call it "short-lived", and game sales for the Wii are perfectly fine.
Also there's no such thing as a "fad for gamers", that doesn't even make sense. Fads are short-lived crazes (and don't last for years), it doesn't get any simpler than that. It doesn't matter what demographic motion controls targets. In the end motion controls have been a big part of this gen with the Wii, Move, and Kinect and have spanned the entirety of this gen. How the market reacts to motion controls in later gens is irrelevant when we're talking about fads.