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.
For AC I'm speaking about handheld games obviously because this game is designed for this kind of hardware and I doubt anyone question the fact it is a very casual oriented game. Then the success of the 3DS version as well as the success of TC proves that despite smartphones casual can still care about this kind of games and buy consoles to play them. It doesn't matter that BT or ND didn't sell well: the fact that casual games like that manage to do well prove my point that you can't conclude that casual gamers don't care any more about gaming systems and proves that they are ok to buy new iterrations of games they liked in the past.
I'm not the one who make big claims, remind you, quite the oppoiste in fact: I'm not saying I'm 100% sure that WS and WF will do extremely well but I question the fact that many seem to be totally sure that WS and WF won't be big sellers. I'm the one who say that maybe they will be right, perhaps, but to be that sure they have to give very compelling arguments and I just don't see any. And if there are no big arguments then the most reasonable thing to believe is that these new games will sell almost as much as the previous ones like we do with every other ip. And when you say "All the signs we're seeing at the moment though, make it seem highly unlikely." I'm sorry but I still don't see the signs you are talking about. Just assumptions that casual gamers won't buy new iterrations of games they liked despite the fact that the same kind of people do that very often to play the new mario, mario kart, AC... games.