The only scenario I can envision involves the collapse of the console market... so this.
I would actually extend the title question to every region. While the outcome in other places is less certain, the current situation is still very favorable to Nintendo.
1) The year head start is more important than most think. Historically, a head start allowed the DC to exist, the PS2 to become a juggernaut, the 360 to keep its head up through a rough first year, and the 3DS to correct it price. Using the most recent fiscal year results from each company, if the next-gen consoles maintained those sales, it would take the other systems almost another 2 years to catch up. It isn't just the bigger userbase, it's the perception of a more staple environment, especially when compared to the other two who will have their own struggles and be going head-to-head.
2) The WiiU's launch line-up is the most mainstream launch the industry has seen. The Wii's launch was defined as either too casual or too niche. Right or not, that's the lesson the industry took from it and that's how they treated the system. Anyone complaining about the WiiU's launch line-up is just ignorant. The system will have nearly double the titles of any worldwide launch, with representation from almost every major franchise relevant in each territory and reaching a broad spectrum of gamers. It will be difficult to pigeon hole the system the same as the Wii when its stop selling games include BO2, ZombiU, AC3, and MH3U off the bat.
3) The Wii lost a lot of potential releases because the system was perceived to not be power enough to make a port profitable. I still expect to hear that excuse next-gen, but not nearly as prevalent. That's means the barrier between the WiiU and the other console won't be as dense. The industry won't as divided. There won't be a PS360, HD Twins... whatever you call it where Wii was the odd-man-out, leper of the industry.