The Wii wasn't and isn't a fad. Nintendo made a bet regarding HDTV adoption and so did Sony. They both sort of half won their bets but each took a bit of a hit for them because of the transition to HDTV. Nintendo bet that HD wasn't a big deal yet and in 2006 it wasn't. Being a more fun and less expensive option, Nintendo basically sold millions. However as technology marched on and as the whole generation moved down the price curve, the PS3 began to be reconsidered. Could it really be a second console at the initial price? No, not at all at $500 for the cheapest model. However at $299 with bundles, it really started to move.
This coincided with the price drops on HDTV's. You can go into almost any store now and buy a 39 inch HDTV for under $300. This was completely unheard of back in 2006. As the cost curve bent, the console best suited for it made the most gains, especially when coming down from the highest prices to reasonable prices.
So it wasn't that Nintendo was a fad. There were other factors. Sony started to execute much better and HDTV's became cheap as heck. In fact part of why Sony is in financial trouble is their television division can't make any money. No one making televisions can make any money. Everything that uses an LCD is becoming so cheap that no one can maintain any price control.
If there is any console that should be considered a fad, it is the Xbox 360. It has bled money continually, it was overtaken by PS3 at the end of the generation and it has been through every configuration possible all just to mostly sell for a small period of time largely in North America. It didn't start with an HDMI port but had one added later. It started with an HD-DVD add-on and dumped that. It started with and has DVD drive in what was supposed to be a next gen HD console.
Xbox benefited from a handful of popular shooters, all of which are largely tired experiences by now. It spawned and now demands the Kinect motion controller on the next generation. It never sold well in Japan. It completely was destroyed in Europe and the rest of the world. It only is in a narrow third place because so many bro's want to play Call of Duty in North America.
After Christmas, Xbox One is just going to get killed because it is clear Microsoft can't sell it well anywhere else besides North America. Wii U is making decent and improving numbers all over the world. Xbox One has already been held even in North America, is losing it's butt in Europe and it is really clear it won't do anything in Japan while PS4 will get a major boost there.
The fad is Microsoft and Xbox. They were the third entry into the console business and have managed to sort of hang around by buying companies and turning their games into exclusives, bleeding profits on their consoles and finally just bleeding even more money by reconfiguring them in every way after placing nothing but losing bets. (No HDMI, HD-DVD, etc.)
They've made a another round of losing bets and this time at the highest price point because they can't continue to let the Xbox division bleed billions. There won't be the same level of sales from them this time at all. They are already the inferior FPS experience. Sony has equaled and surpassed the experience of Xbox live along with offering games at a better value. Both Sony and Nintendo offer much better values at high and low price points. Microsoft is just going to get crushed and proven to be a largely financed and one time fad.