All going to boil down to the games.. Not the marketing, not the distribution, bundles, price drops, not teh "cazuals", none of that bullshit. These factors may sway things a bit in one direction or the other, but at the end of the day, It's ALL going to boil down to the games. Why do you buy consoles in the first place? What is the SOLE purpose of gaming consoles? (Hint.. in aint to watch netflix, jerk it to epic graphics, or waggle a wiimote). Sure you might get off on horsepower, or gimmicky controllers. But at the end of the day, what does this horsepower, or these gimmicky controllers PLAY? GAMES.
Wii U could get Dreamcast-like sales, or it could get SNES to near-Wii sales. ALL of it boils down to the quality, and kinds of games it gets.
I say, if we get more games like Zelda WW, Pikmin 3, Wonderful 101, Bayonetta 2, etc (nothing against these types of games, but they are incredibly niche).. We end up with Dreamcast or Gamecube sales (15-25 million).
However, if Nintendo goes more in the direction of NES/Wii style games (the kinds of games that made them such a phenomenon) like NSMB, Smash Bros U, Mario Kart 8, Wii Fit, Wii Sports Club, Wii Party U, DKC.. We will see sales trend more towards SNES level (40-50 mill)