I'm not saying that the Mario Audience will dry up. I never even mentioned Mario.
You suggested that Chase Mii and Battle Mii weren't fresh or new enough in order to entice people who enjoyed the experiences Wii provided them. And that's an awfully similar argument to what people say about Mario - that it doesn't change enough so why do people keep buying it? As in Mario's case, the next step for the Wii Sports crowd needn't be a revolution, but an incremental step towards a new implementation for an appealing core experience.
I'm simply saying that they won't get the WiiSports crowd back with Find Mii. It does not have the same novelty.
Here is what made the Wii successful, IMO:
1) It removed barriers to gaming by shifting controller paradigm.
2) It allowed people of wildly different ages and gaming experiences to play together.
3) It gave families and groups of friends the opportunity to see gaming as a spectator sport - ie. it allowed you to watch your uncle, aunt, grandmother, and grandson behave in an uncharacteristic way. One of the most successful multi-viewer experiences I ever found for my family in particular was Wii Fit. Even if it was mostly a single-player title, people loved watching others play it. It was hilarious and socially engaging.
4) Nintendo legacy IPs, yadda yadda.
Chase Mii and Battle Mii hit all of these, especially after some of the basic training gained by the Wii. You are seriously underestimating the premises of those test games to suggest that they won't appeal to a large audience. Even core gaming sites mentioned the inherent appeal of Chase Mii, in particular.
Especially when you consider that you can only have one or two people with the fancy new controller at once. It's just not going to hold the same appeal.
IMO, the only people who will bitch and moan about the fancy new controller will be traditional gamers. The mainstream just wants to have a good time with their friends and families - we're the only ones who get bent out of shape regarding the tech in our hands.
As for what they missed to go back to their roots, they needed to invest heavier in their own platform, something they did a lot on everything up until the Wii. It was just mind boggling how they sat on it for so long.
This I agree with, but there's nothing inherent to the Wii design that made this occur. Nintendo simply fucked up.
ANOTHER NOTE: I friggin' love the asymmetric nature of the Wii U. Frankly, I think it is compelling as hell and allows for multiple sorts of experiences with a single piece of software. That is genius, from my point of view. Color it as rationalization or no.