The thing about someone copying Wii two or three years down the road is that Wii would have hundreds of games and probably tens of millions of users. Why would you want to buy an imitation with only a handful of games? Let's call the imitation T-Bone.
1. T-Bone has better graphics, oh no!
But if Wii is already such a big success that someone is releasing a whole new system to imitate it, then we know for a fact that better graphics aren't much of a selling point since PS3 and 360 had noticeably better graphics all along! T-bone would also have to cost more than Wii, and again, its game lineup would be pitiful by comparison - T-bone is to Wii as 3DO was to SNES and Genesis
2. T-Bone has better motion capture, oh no!
I notice that the Fusion thingies try to boast superior motion capture abilites - they'll work without a direct line of sight, and under any lighting conditions. However, videos show that they have just as much lag as Wii (if not more) and they probably have other issues we don't know about. Instead of light interference, maybe you need to worry about sound interference. At any rate, T-Bone would need to feature much better motion capture than Wii before it would really matter, because consumers are still looking at T-Bone's 10-20 game launch lineup and wondering why they should buy that instead of Wii.
3. T-Bone has a better Virtual Console...oh wait, no it doesn't. Nobody will ever have a better virtual console, at least not for the forseeable future, because Nintendo has the best game lineup of any developer in the past 20-odd years, and it has secured the support of many other key developers.
Basically Sony and Microsoft are out there now trying to convince us not to invest in Wii, and they're going to use this development to try and scare you, but we shouldn't be scared. Right now the scare tactic is, "A Wii killer is right around the corner - look at these!" A Wii killer is not impossible, but it needs to come to market by next Christmas if it wants to be effective, and I think Sony and Microsoft are still too busy fighting each other to bother. A surprise contendor is possible, but not probable. Nobody but Microsoft has made a serious attempt at entering the industry since Neo Geo pocket and Wonderswan died.
/end FANBOY.