You are making judgements about the quality of Wii games and assumptions about Wii owners which you can't back up and which I would argue don't even matter. You may not appreciate many of Wii's offerings and you are entitled to your opinion but this 'quality' doesn't speak to the buying habits of the people who play Wii games or their likelihood of supporting Nintendo's new console. You are using soft data (surveys?) which suggest that these people don't play their games much but in the end it doesn't matter one bit as long as they buy games, and by extension, hardware.
The only hard data we have is attach rate. You are right that the attach rate doesn't mean much when you take a snapshot but (and you'll have to take my word for this if you are new to the site) the Wii's attach rate has been rising for over 3.5 years which tells us that it's established users and new users are continuing to buy games in significant numbers. Once again it doesn't matter if they only play each game for 5 minutes, the point is they are willing to hand their hard earned over to Nintendo /andor it's third parties. So what about the behaviour of these people makes you so sure they won't continue to spend their money on Nintendo in the next generation?
The only data we have says they have been willing to buy games and continue to buy games, which strongly suggest thats they have so far been happy with the products they have purchased and are willing to spend money. I'm not saying this guarantees their future loyalty to Nintendo or their gaming habit, but why are you so sure that it doesn't.
Well let's look at thte facts of the wii and what a new one could offer:
The wii is technically very dated. The games are graphically dated and AI is not going to be impressive compared to the rest of the market.
The pad response time is about 180ms meaning the owners are not concerned about a responsive pad, nor is it 1:1 without motion +.
What could a new console offer that could entice people who clearly are not bothered about graphics or tech as they have the wii to begin with?
If they wanted cheap gaming they'd have chosen the xbox. If they wanted hardcore games, xbox or PS3. If they wanted state of the art graphics/bluray they'd have gone for the PS3.
So again, what could a new console possibly offer to these people who dont care about graphics, tech or media?