1) The 3DS isn't more powerful than the Wii.
2) You said it yourself that the 3DS is a new HANDHELD. It is not a replacement for the Wii CONSOLE. Granted, a new Nintendo console may be not far over the horizon, though...
3) 3rd Party support be damned. It was never really there to begin with, now was it? Yet the Wii has sold over 70 million units to become the best selling Nintendo console of all time. The Wii isn't anywhere near death, and that's true even if a new Nintendo console happened to be announced in the near future. Look, the GC died quickly after the Wii came out, but the GC had a very small install base. You notice how the PS2 held strong years after the PS3 was released and how it is still getting new titles to this day? That's what we will see with the Wii even after the Wii 2 comes out, because its in too many damned homes for it to die off immediately. Plus many A+ titles are known to be coming soon (DKC, Goldeneye, Zelda, Wii Party, etc.) plus many more that are yet to even be announced. The Wii has a lot of life left in it still.
4) I also kinda think the Wii controller could be carried over to the next Nintendo system. There really isn't much need for Nintendo to design a new controller from scratch, so this Wii Mote Plus could be the standard controller for the Wii 2 just like how it is for the current Wii. Why the hell not? Of course, a Wii successor is probably a year or two away anyway. The DS came out in late 2004 and the Wii came out in late 2006, so there was a two year gap between those two systems. History might repeat itself now with the 3DS release being imminent, but the next console being another two years off. I don't think we need to worry because we don't even have rumors about it at this point. You know its far off when there's not even rumors.