1.) The Casual Market is too unpredictable to say that they won't upgrade.
2.) It's very likely that the Wii's successor won't be called a Wii 2 (Taken that Nintendo has never been inclined to use 2 or 3 in the name of their consoles.)
If the Wii's successor is marketed in a similar fashion as the Wii and shows enough enhancement from its predecessor it should sell strongly. I think if the Wii's successor secures stronger third party and enough interest that it's just for "non-gamers" and "casuals" then it should sell more, provided the 360 and PS3's successors don't directly compete with it. If they do, I don't think it will.
EDIT: This may piss people off but kids are going to remember the Wii and talk about how awesome it was in their teens and earlier adulthood. I think these will likely buy a "Wii 2." Also a strong part of the casual market is children which are more likely to want the new "bigger and better toy" as soon as it comes out then others.
Pixel Art can be fun.
