Well since you asked so nicely...
1. The Wii is a fad argument has little merit. It IS certainly fair to say that the Wii's fanbase was less devoted than the XBox 360 or PS3, but fad is unwarranted. Firstly, the Wii sold well for as long as you'd expect any game console to do. The Wii sold well above its competitors until 2011 which was 5 years after its launch. Sales dipped below their competitors when Nintendo stopped supporting it with software, which exposed their massive weakness in third party software. But, it had an average video game console lifespan. It has a chance to go down as the second best selling home console ever... so I don't see fad here. Fad implies it had a short period of popularity, which wasn't the case.
2. The idea that Wii owners never bought software after buying the system is clearly erroneous. While the Wii's tie ration IS smaller than the 360 or the PS3, it is only slightly so. 9 compared to 9.6 for the PS3 and 10.62 for 360. That number is a bit inflated by the fact that the Wii was always bundled with a game of some kind. But, even so, the idea that Wii owners never bought a game beyond the initial purchase is clearly false. On average, they bought 8 or 9. The Wii still has moved the most software according to our figures, and while the 360 will eventually overtake it, the idea that Wii owners never bought games just doesn't fly.
As for the Wii U's overall success, I'm a bit too tired to flesh out an argument, but I'll say this. Everyone likes video games. Gaming isn't a 16-34 year old male thing, nor is it a little kid thing. The thrill of getting a high score is universal. The idea of exploring and prodding an interesting new world is universal. The idea of competing with others is universal.
The problem is that gaming has evolved in such a way that it only appeals to a very narrow segment of the population. Companies never invested in building a market outside of 16-34 year old males, with a smattering of kids as well. Because they never nourished these potentially fertile markets, it's very hard for them to make a profit in them, so they quickly return to what they know. Shooters, sports, and open world games.
The Wii succeeded because it took the universal love of gaming and combined it with the right software. Wii Sports allowed the thrill of competition for people who can't manipulate 2 joysticks and 8 buttons. NSMB brings gaming back to a simpler time when rules were clearly laid out. No inventories, no manipulating a 3D environment, and no grand puzzles. Super Mario Galaxy was a game that for all its inventiveness is played mainly with 3 different inputs and an analog stick. Animal Crossing provides the joy of exploration without the fear of defeat, Just Dance allows amateur dancers to get their groove on, and so on so forth.
Basically, Nintendo needs to provide great "casual" software to attract casual gamers, and they haven't done that yet. The Mario games and Wii Party U are a good start. A packaged Wii Fit U and Wii Sports should be good. Nintendo's Karaoke service could be a nice boost if properly advertised. Mario Kart will possibly be the biggest Wii U title when it comes out, and Smash will be nice as well. These games will help ensure Nintendo has relatively decent success going forward, but Nintendo also needs exciting new casual games to invigorate their market.
The good thing for Nintendo is that Sony and Microsoft have thus far shown exceeding disinterest in the casual market this gen. The casual market is still Nintendo's to grab. It is still 100% possible for the Wii U to not only match but exceed the Wii's sales. Of course, it's not likely based on Nintendo's current trajectory, and every passing month makes it harder. Still though, if Nintendo creates the right software, they'll succeed. If Nintendo doesn't succeed, then we'll be left with Microsoft and Sony fighting over PS360 fans with both new consoles struggling to expand the market.
