If third party devs make games that people want to buy, then they'll get a Wii or whatever console. I'm not inclined to believe the average Nintendo fanboy is any more discriminating than the average Sony or MS fanboy (as far as first party titles go).
A lot of the people buying the Wii right now are a different demographic entirely, and aren't going to be buying as many games as the 360/PS3 buyer will-and when they do buy a game, it'll in all likelyhood be a Nintendo one.
At this point, I would agree, if only for the fact that there are hardly any noteworthy third party titles have been announced on the Wii for people to bother getting excited for.
The point I'm trying to make here is that if/once a steady stream of third party titles start appearing, then those who aren't interested in Nintendo games will eventually get it.
Sure, Xbox360 games sell great. But come on, let's at least try and be fair. 360 gets Oblivion, Gears of War, and Lost Planet while Wii gets...SSX Blur? The reason you see Nintendo titles dominate most of the time is cause they're the ones that are usually done with any effort involved (yes, not all the time, obviously). Therefore, it only makes sense that the only game comparable to those 360 games that I mentioned is something like Zelda.
Then if you compare something like PS2 to GC, well then you're comparing a system that has over 3,000 games to one that has probably less than 500. Can you blame the demographics then?
Userbase --> Software ---> userbase. This has always been the case, and most likely always will be.