I don't really understand the reasoning behind most of the arguments that the Wii will (suddenly) die off in a couple of years. I don't expect the PS3 or XBox 360 to be inexpensive enough to really tap into the mainstream buying market until 2009 or 2010 because most console purchases are made at $200 or less; this means that for the next 12 to 24 months the Wii is (essentially) without competition because the XBox 360 and PS3 will be too expensive.
As the PS3 and XBox 360 come down in price the Wii will also see lower prices and/or increased value from bundles; when either systems hardware is $300 you could see a $200 Wii that is bundled with a high quality game (Mario, Zelda, Mario Kart, Smash Bros, or Metriod Prime) with 2 Wiimotes, 2 Nunchucks, and Wii Sports which I expect will be seen as a screaming deal. When the XBox 360 and PS3 are approaching $200 the Wii could be $150 or $99 and there will always be people who are willing to spend $99 on a gaming system regardless of the quality of graphics.
Even if you do assume that as the PS3 and XBox 360 hit $200 people will suddenly become (far) more interested in them, and the sales of the Wii start to falter the likely time for this to happen will be 2010; to a certain extent this really plays well into Nintendo's hands for the following generation because they can produce a system which is halfway between the PS3 and PS4 in terms of performance, is cost effective, and release it in 2011 to an existing userbase of Wii owners who are looking for a more powerful console.
