Yahtzee, this is why I respect you so much. Even if you are fond of sweeping generalisations and hurling abuse at developers, audiences at critics alike, there's a strong underlying reasoning behind it all, and you've expressed it exceptionally well here.
Personally, I hope that the next mainline Mario game will continue to innovate again. While I've greatly enjoyed both Galaxy and Galaxy 2, I'm quite aware of the "expansion pack" nature of it, and have been carefully ignoring all the hand-crafted hand-holding Nintendo has packed into the game. It's much better to play in a relaxed, one-hour-per-day way I've found.
Mario certainly isn't casual, but Nintendo is determined to make their idyllic polo-shirt wearing happy families cross the gap into more complex 3D gaming.