Headstart as in
- know-how of developing games for a wiimote controller
- massive third party support just because they're the market leader (it'll be there for sure)
I think the central problem with your premise that the Wii will hold onto the casual gamer audience is that... the casual gamer audience doesn't give a whit about brand loyalty. Software's king. And, like you said, the casual audience doesn't -really- care about all the big-name games we do. What Sony needs is an affordable system and... shovelware. The Wii is remarkably easy to develop for, but we have no idea what Wii 2 will be. And really, there's nothing remarkable or unique about the Wii * branded 'lifestyle' games, nothing Microsoft or Sony won't copy beyond heavy, heavy marketing.
Normally you wouldn't think they'd screw this up, but this is the console industry, it wouldn't be the first, or even the second (or even a third, really) time a market-leader screwed up a monolithic lead.
I can see shutting down SCEI as an option to short-term profitability, but I think it's far more likely that we're going to see a retooling. And no, the Wiimote isn't the key to it nearly as much as marketing is, which was Nintendo's brilliance this generation.
And again, though Wii Music isn't a
flop it still hasn't shown the same performance as its older brothers in the Wii * line, and that says that Nintendo certainly isn't infallible. To me it says they haven't cracked the nut of this audience, though with heavy, heavy marketing they can make up for much of that.