Probably when Wii reaches 20M+ it will draw more 3rd parties, the gamecube did not do as well as the old Xbox or Ps2 and 3rd parties went away. It will take time before they come back and only when it is obvious they can make money back on titles.
Should be pretty clear by now that Nintendo is back in the Green.
Other problem which Wii and PS3 also has is the motion controller. Developers are new to this and in my opinion pretty slow to implement any but the most basic form's of use for this new interface.
On Ps3 for example many games are still using only the thumbsticks , and where motion is used it's sometimes quite minimal and often badly misused or sometimes flawed (Bazing angles 2 demo uses multisample smoothing on the motion steering which is ok but i noticed when framerate misses the steering suffers) .
IMO developers are having alot of trouble with motion because they are an inexperienced bunch of amatures. , in wii bowling you can cheat by flicking the controller, in lego starwars they completly stuffed up the lightsabre motion etc.
Anyway 2008 may see an improvement.
PS3 number 1 fan
