So many assumptions being made here ...
Third party publishers and developers do not live in a vacuum, and they're not completely devoid of independant though and intelligence. They will all act on their own in an attempt to maximize their personal results and profits. Some third party publishers and developers may "ditch" or avoid the Wii because it does not suit them or their larger strategy (Epic Games would be an example of this), while most will see the large userbase and lower development costs as a way to get the best return on investment in order to maximize their profits.
What does this mean in terms of games?
Some publishers and developers will see the Wii as an opportunity to tap into a new and growing market segment which they have been unable to tap before, while others will realize that the Wii is (currently) being bought primarily by conventional gamers and they will try to market towards these gamers. This means that the Wii's library will probably grow with lots of diversity ...
In contrast, I suspect that the PS3 and XBox 360 are likely going to see less diversity from third party publishers and developers in the future because the high development costs will force publishers and developers to avoid taking risks. Publishers are going to look at the fact that (pretty much) the only games that are ensured to break even and turn a healthy profit are First Person Shooters, Racing Games and Sports games that are released on both platforms. The days of the quirky game produced by the small developer will be over for the Playstation, and there will be mountains of games about space marines fighting off aliens in a destroyed urban environment.
