I dont think the next Wii console will be weak at all, if it is i predict it to fail miserably.
Sony and Microsoft both are going to allmost surely go for powerful consoles, and they can easily have motion controls that are far superior to what the Wii today offers.
If Sony & Microsoft also have motion controls, + much better technology, i dont see how the Wii will compete in the long run. The prices differences between the Wii and the cheapest Sony\ microsoft offering would have to be huge for anybody to keep buying a Wii.
If you get the choice between Sony\Microsoft offerings, which haves the same control scheme if wanted, plus all the other features MS\Sony consoles have + better graphics, only kids will buy a Wii.
Then then again, that may just be the case today, that a lot of kids buy\want Nintendo Wii's. Atleast when i was growing up, Nintendo allways was the most kid friendly console by miles.