i don't think hardware, online, HD, or 3rd pary support would be reasons the wii "fails". they're not reasons the wii is doing well to begin with, so i doubt they will be reasons for wii's "failure". the biggest concern is if devs never utilize the wiimote past the "demo" stage, by "devs" of course this includes nintendo itself. the 1st wave of wii motion sensitive games have been so far unsatisfactory, like TW07, SSX and Cooking Mama. the wiimote will risk becoming a gimmick if the trend continues, and that's what will truly reduce sales. so in a sense it has to do with 3rd party support though it's by consequence. for 2007, the best motion sensitive games are surely gonna come from nintendo itself. let's see just what you can do with the wiimote other than tennis, bowling and baseball.
the Wii is an epidemic.