the only thing stopping the Wii from selling well is the controller and 3rd party support. What they are going to lose is probably cross platform games. Nintendo will have to make up for the loss of these games, ALSO, there are going to be some games that you just can't use that controller for.
I think the major focus is going to be on the controller. I've used motion sensing controllers before and they aren't very good, often very iratic, so I hope the technology has been changed and perfected with the wii, because nobody wants a system that every game plays crappily because of over sensitive motion sensing. And because nobody really has used it, you don't see to many reviews, but the feeling at E3, at least from G4s perspective, was that it was a mixed bag, it worked well on some games, but other games, like Metroid, they said it was pretty hard to use, other than for aiming.
But again only time will tell. The wii will sell very well at first. I'll wait to see how well it sales after the first year and i would also bet the wii gaming library will be thinner than the Gamecubes, which i wouldn't be suprised if that isn't the reason for the Wii being able to play downloadable classic games. But again, only time will tell. I don't think the launch is going to be as important as where each system is a year after.