I agree that third parties are being lazy on the wii, and that's why they aren't getting 5 million sellers, but also there is the fact that everybody is busy buying nintendo no matter what game it is. They poop gold.
Games by Nintendo are selling the most on the Wii, but I find "Nintendo has put the most effort into their Wii games" a much likelier explanation than "Wii owners don't want to buy anything except Nintendo's first party games".
Remember, there has yet to be a third party game to sell seriously less than expected on the Wii, and the amount of games with a lot of effort behind them is several times larger this year than 2007.
Sadly this is one of the last straws Wii haters have, so we'll be seeing a lot of it in the near future.