First off, The Wii makes money with each console they sell, whereas the PS3 loses cash and i'd be willing to bet the 360 doesnt make much. So even if the wii sells less games, Nintendo itself is still profitable.
And who says casual gamers buy less games? A hardcore gamer may buy 5 halo-like games, a casual gamer may buy 5 mario party type games. Either way, there's money made. I really doubt casual gamers buy their console, buy 2 games, and then just stop looking at new games. That realllyy wouldn't make sense, as long as companies keep making games aimed at casual players we'll keep seeing game sales.
And if we're gonna have a problem selling games... why are Zelda and Paper Mario outselling pretty much everything? That doesnt make much sense, does it?
And the Wii has hardcore gamers. Just because the target market has shifted doesnt mean some serious gamers have gotten along for the ride. If they hadn't, we wouldnt see games like RE4 and Manhunt being developed.