I think Legend is right to an extent, as a lot of publishers are risk aversive and would rather stay with what they know works despite the deminishing returns.
But I think a game like TimeSplitters 4 or Monster Hunter 3 coming to the Wii and doing very well, will change a lot of other publishers minds. Inevidably some publishers with some vision will blaze a trail on Wii and others will follow. Wii will become the main focus for 3rd parties (in general) but PC/360/PS3 support will never go away entirely. However they'll get doomed to even more of the same as, again, 3rd parties would rather drop CoD5 - 7 on them knowing it'll sell, than do anything like NiGHTS and risk huge losses.
BTW I think NiGHTS will do fine on Wii long term like many other games that started slow (Carnival Games, My Sims, Zack & Wiki, etc.)
