Ehh...I'm starting to get the idea that you're dealing with a tautological argument here. What comes first: the marketshare or the third party support?
The Wii is an interesting scenario because people continue to predict its imminent demise, no matter how much the console continues to sell. Nevertheless, despite the ample amount of third-party software on the system, it appears that many of them are not selling. Of course, the choice between Metroid 3 and...whatever Wii shovelware is an easy one, but why didn't people buy Madden for the Wii? The counter argument is that *good* third-party games do sell, but I don't see much to support that other than Resident Evil 4 (I'm also not convinced that the consumers really can tell the difference between "good" and "bad" games).
So, the Wii is selling like mad at the same time that it seems rumors abound about developers and publishers getting cold feet about the system. It's a strange phenomenon, to be sure. I can't help but think about my belief of the Wii as a "supplementary" console for gamers...but there's little to back that up at this point.