none of these systems "NEED" anything else. it's the games and the power of ideas that really keeps things going. it's the same exact thing when it comes to movies. Yeah cool special effects and high paid actors are good to have in a movie, but what REALLY matters is the story and entertainment factor. If the script and story sucks, then the movie is going to suck regardless of how many special effects and big stars are in it. Case in point, Napoleon Dynamite did phenomenal, despite being made with virtual unknowns and on almost no money. But movies like Bloodrayne, Adventures of Plutonash and Gigli all suck so hard.
Same goes for games and systems, graphical power and speed are all well and good, but if the games aren't rocking then the system is essentially just one expensive paper weight. You need awesome games, no matter the graphical beauty, to keep your system alive, and that's what the Wii has. Not to say the Xbox360 doesnt have them as well (i've yet to see any real ones on PS3) but I'm just saying the Wii has what it takes to go 5+ rounds. Especially if the developers take advantage of its unique capabilities. I'm sure it'll take a little while for them to figure out some nifty stuff to do with it, like it also took them a while with the DS but it will come with time and experience.