I don't think that it is plausable that the Wii would "Fail" being that it will probably break 15 to 20 Million unit sales by the end of 2007; at this point (even if sales slow down to Gamecube levels) the Wii will have decent support for the rest of its expected lifespan.
Now, as I see it there are many risks that would force Nintendo to underperform in 2008 and 2009 but the main ones would be Nintendo not comming up with inovative new games that use the system and third party developers not heavily supporting the system.