I think you're failing to understand what made the Wii so popular.
It wasn't the presence of motion controls so much as the absence of them in the other two consoles. Had the PS3 had the motion controls and affordability of the Wii, together with the Sony brandname, at launch, being a successor to the PS2, it would be in first place right now. Particularly if the other two consoles featured no motion sensing. But even if they did, because of Sony's power, the PS3 would be in first.
But, of course, that's not the case, and the Wii launched with a low pricetag and motion sensing. Because people wanted something new, and others wanted a reason to get into gaming, the Wii was a massive hit. Putting motion control into the Wii (or 360 for that matter) would do nothing. The Wii is associated with motion sensing in the minds of the public, they know the Wii had it first, a lot of people already own Wiis, and you can get, in the eye of the casual gamer, the same offering (motion sensing) for a far lower price on the Wii. The 360 may be cheaper, but any attempt to chuck motion sensing in would be seen as desperate and pathetic, and would have little to no effect, because it's not novel anymore, and Nintendo has had time to improve it.
So in short, it's too late to do anything but ride this gen out and hope for second place and, more importantly, profitability.
(Former) Lead Moderator and (Eternal) VGC Detective 
