Your reasoning is the same thing I've heard before. What it comes down to is if they have the right games and (to a lesser extent) price.
The power issue is not really as big of a deal as many think, no matter what the final specs are for WiiU. Even if the games just look around the same as the competition to most people, there are enough Nintendo fans to make the launch a success (unless it's prohibitively expensive, which is highly unlikely). If they have the right games (particularly ones that prove why the controller is neat) that make everyone want one, that's where they will easily amass a 10-12m+ install base before the 2013 holiday. With Nintendo's strong IPs and that big of a lead, it would be difficult to imagine WiiU not doing well in the 8th gen, even if the competition does well too. So E3's revelations of games and price will tell us this. If we see even close to the E3 '06 hype level, we know it'll begin strong.
One point you are wrong about is to say that casual gamers are gone. A casual gamer is nothing more than someone seeking a fun experience that's "in" right now. If WiiU has the new games that everyone is talking about... and is the only place to get these games... that is where the casuals will bite. Remember, WiiU won't be $300 forever.
Concerning NeXtBox and PS4 we know nothing yet, but I will say that if Nintendo continues to make games that the market wants, WiiU will do well no matter what the competition does. But in addition to this, if it does indeed acquire the 3rd party games Nintendo says it will even after the new consoles hit (even if they don't look quite as good), then the competition may be the ones in trouble.
