A lot of people doubt the Wii because its lack of HD and advanced graphical capabilities, at the same time I believe people have overestimated when HD would really be a selling feature.
Microsoft and Sony bet heavily that between 2006 and 2008 HD was going to be such a selling feature that most people would refuse to buy any product that wasn't sold as being HD; I say "sold as being HD" because as recent games have demonstrated both the PS3 and XBox 360 have difficulty producing games with the advanced graphics while maintaining a decent framerate at true HD resolutions. As we know, this is not how things turned out and people are (for the most part) entirely happy with buying standard definition products regardless of whether they own a HDTV or not ...
Now, this doesn't mean HD won't ever be as important as they envisioned but I suspect their timing was off by (roughly) 5 years. Right now it is pretty difficult to buy a TV which does not output at HD resolutions and most people who buy a new TV from now on will get a 1080i, 720p or 1080p TV, and most people who don't currently own an HDTV will have to replace their TV in the next 5 years which will result in most households having one or more HDTVs. On top of this the move towards Digital Broadcast, cable companies offering more HD channels at a reasonable price, most TV shows being broadcast in HD, and the slow adoption of Blu-Ray will result in people becomming accustomed to HD signals; this will make people more aware of the difference between standard definition and HD and make HD a bigger selling feature. All of these factors will combine together and HD will become the massive selling feature people expected it to be between 2012 and 2013.
Basically, the assumption that HD will (suddenly) become popular enough to kill off the Wii is false and it will continue to sell and have a long healthy life. Being that the Wii is selling at a faster rate than the PS2, and that it may be able to maintain or increase its pace because of the growth in the industry, it may pass 100 Million units sold in 4 years and could possibly sell 150 to 200 Million by the time Nintendo stops making them; this would mean that the Wii would easily pass the PS2's total.
