The wii sold alot right out of the gate. I never use mine, but I think the market is probably saturated by now? They sold so many of them, so quickly, that they'll probably hit that point where everyone that wants one has one.
I think the Wii will have a hard time selling the next version of the Wii, as the Wii marketed to alot of casual players. Players that didn't care about deep games or HD content. The same type who might not care if theres a wii upgrade.
As for the 360/PS3, there both pretty good consoles. I still think the 360 has a bit of a deeper catalog, but the ps3 has made alot of ground the last few years. For hardware I still feel the ps3 is more bang for your buck, with bluray built in and free online gameplay.