If the Wii U manages to sell 20 Million+ like the GameCube did, they will likely just release another System in 2017 and move on.
If Wii U fails completely as in pulls a Saturn or Dreamcast and manages only 8-10 Million lifetime, I'd assume that Nintendo would stop making Home Consoles, and focus on their Handhelds. I personally don't think that would ever happen though.
Plus, unlike back in 2001, a lot of people buy more than 1 System per gen. PS2 completely dominated while the GC & XB were left with only the Deepest and most dedicated fans. Wii wasn't ALL Casuals. People had a PS3 + Wii or 360 + Wii. Sure, it wasn't their main System but it was their 2ndary.
Once the Wii U expands its library, it will start selling fine IMO. Also, not just Mario or Zelda. I'm talking other games too like Kirby, Animal Crossing, Pikmin, Starfox, The Wonderful 101 and many more. Sure they don't sell AMAZING on their own but they still have a following and can still help sell Systems. I am a believer of library's selling Systems not individual games. I could be wrong but I expect Wii U to also be the 2ndary System for a lot of people because of the exclusives. Orbis + Wii U or Durango + Wii U is likely IMO.
