I think the Wii U has a better shot at winning core gamers back than they do at luring casuals into buying another console.
Part of it is that most casual gamers won't feel the need to upgrade. In their minds, they bought a Wii so they're good for several years. I don't think that we'll see a casual boom in back-to-back generations. Most of them didn't buy it for cutting edge graphics or so they could grab the newest release, but rather for one or two games, usually a fitness or party type game that they played at someone else's house.
There is also the focus of the Wii U, the tablet controller, which actually is even more susceptible to the problem standard controllers had with regards to casuals, which is that they look intimidatingly complex. With the Wii's wand controllers, any novice could jump in and play. They were simple and intuitive, which is perfect for people who just aren't going to take the time to learn something new. The Wii U controller probably won't appeal to non-gamers in the same way.
The Wii U might "win" in the fanboy metric of most units sold during an active generation but I think the sales rate is going to fall back from the Wii (excluding the Wii's final year or so). However, if Nintendo continues to put serious effort into pleasing the core gamer, I think they have a good shot at the Wii U having a much higher rate of satisfaction in that demographic, and that's the demographic that buys lots of shiny new games.
