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I think it depends on whether or not all the soccer moms/women that bought a Wii for Wii Sports/Fit come back to the Wii. My personal feeling on this in the long term is "no", not happening. This audience is now happy with the iPad/iOS devices they have and the $1-$2 app software model, "Wii" as a brand is yesterdays news to them.
Mario + Mario Kart isn't going to carry Nintendo to 100+ million units.
Maybe Wii U takes off in Japan, but even Wii really did not take off in Japan after that first year craze. Do the Japanese even want a console anymore? Seems like 3DS/PSP have filled that need for them.
The demographic that they really need is that non-gamer/women demo, and I think a lot of them either have moved on or simply aren't interested in the "upgrade to a new system!" model. Wii was fun for them for what it was and it had its moment in 2006-08, but I think a lot of these folks have moved on.
That leaves hardcore gamers, and I don't see many of them abandoning MS/Sony for what Nintendo is offering -- a bunch of PS3/360 ports and one Zombi game exclusive. For Nintendo to seriously compete against MS/Sony here, they'd have to roll out huge dollars to secure something like GTAV as exclusive and then have a gaurunteed stream of core content 12 months of the year to go with that. Realistically that's what it would take IMO to gain a chunk of that audience.
My feeling is 45-65 million (SNES-NES range).
