And that matters why?
You don't think that people won't be willing to pay twice the price for more than twice the functionality? By the time the PS3 and 360 are $299, their feature set and library will be filled with utterly ridiculous titles that will sell like wild fire, plus an amazing 'Greatest Hits' catalogue.
What bizzare world are you from, btw, where fads don't last for 8 months? Please try to tell me that more than 50% of Wii owners play anything more than the minigames available on the system. Please. It's almost a sure bet that the people who bought Wii right off the bat will not be the same people to help keep the system afloat with software sales. Try not to confuse the North American Market with the Japanese market.
Also, the DS v PSP is quite a different world. The DS and PSP are both very different, the PSP being more so because it's more "mature" as both a product and it's software. Not to mention it's more expensive. But that still doesn't stop the PSP from being as successful as the PS2.
My point is, it doesn't really matter if the Wii keep selling, or doesn't, what does matter, however, is that the sales will slow a lot once the PS3 and 360 hit price points of $299.
I'm going to have to disagree with you pretty much across the board.
When the Wii-mote was shown at E305 (I think it was 05), it wasn't the promise of making games simpler to pick up to play, or the mini-games, or Nintendo franchises that got the attention of the gaming world. It was the idea that it could open up new control methods such as sword fighting, and bring FPS on consoles closer to their mouse and keyboard PC counter parts.
It was like a huge brainstorm that went through the gaming world at once.
So, I'm willing to bet that many Wii owners are the hardcore type. I'm also willing to bet that they're waiting for more hardcore games as well. It doesn't mean smaller games won't sell, as hardcore gamers play those sometimes as well, but they're waiting for games with "meat" on them.
One piece of evidence is Resident Evil 4's reception on the Wii. It's supposedly sold a lot (we'll find out exactly how much this evening), and it's the same with Dragon Quest Swords in Japan.
Is there a new audience? Sure! But I don't think it makes up a majority. In fact, I believe one website (I forget who) said that only 10% of Wii owners were new gamers. While 10% is awesome for expansion of the industry, that still leaves another 90% who have played games before waiting for more stuff.
Also, by the time the 360 and PS3 are $299, the Wii will likely be $99. So really, it won't matter. By that time, a lot of people will be able to buy more than one console, and everyone will be able to get a share of the living room.