AC is still selling amazing well which proves that the casual are still interested in this ip not only the Nintendo fans. BT, Nintendogs have a lot of ripp-offs on smartphones while the only games that can compete against WS are on other expensive consoles and those games didn't have the same kind of success like MK clones never overshadowed the original. Speaking of WF, it really has no competition, Zumba being the closest one.
So the situation seems very different to BT and ND and at least there is no proofs compelling enough to claim that these particular ips will have the same fate as BT and ND and not continue to sell really well like other casual ips like AC, TC or even MK.
I still don't think this Animal Crossing comparison holds any merit.
It then went on to sell ~ 4.5m on Wii, despite the massive disparity in install-bases between GC & Wii. While I have no doubt it's the casual friendly game you're making it out to be, it really didn't do much to attract a lot of new fans in.
What happened with Animal Crossing was a portable explosion. It found its home on handhelds. It felt like a game that was designed for handhelds - ideal to play in short bursts where you can pick some fruit for 20 minutes here or dig up some weeds for 10 minutes there. And that's carrying on with the 3DS.
So yeah. I'm still not quite sure why you keep bringing up this Animal Crossing comparison. Wii Sports / Wii Fit, in my mind, bear a lot more similarities to Nintendogs/Brain Training in that they were explosive new IP's which came out of nowhere and attracted a whole new crowd of people to gaming; were imitated by everyone under the sun and (potentialy) became far less successful when they returned on new hardware.
Do you have any thoughts on why these casual gamers would upgrade from the Wii/Kinect that they're already playing on to a new console?
Maybe you're correct. Maybe the casual gamers will flock back to Wii-U once Wii Fit and the (potential) Wii Sports release (remember, they haven't announced anything about that yet). All the signs we're seeing at the moment though, make it seem highly unlikely.