I don't really think any of the consoles are in danger of going. Wii may be slowing down, and it's true that casuals are less likely to go buy software, but as long as they introduce a few huge casual traps a year and sell 20 million plus copies, they are going to be fine.
On the other hand, I think Nintendo is in the worst position for moving into the next gen despite their huge amount of success. I would guess that the huge majority of 360 and PS3 owners take gaming seriously enough that they would definitely replace their consoles when an upgrade is released. Nintendo's huge market share is the result of capturing casuals in droves, and a large part of that is because the Wii was something new and novel. I'm not convinced, though, that they have captured the casual market to the point that a large majority of their users would go out and buy a new Wii2 or whatever. They struck gold with the Wii, but I don't know how many of the casuals they lured in are actually interested in consoles enough to go spend a few hundred more dollars to replace the Wii which, if software sales aren't lying, they aren't going out and buying a large variety of games for anyway.







