Remember guys, Metroid Prime was a spinoff of Metroid.
Oh, and by the way... People don't buy Nintendo's systems for Metroid; they buy the systems for Zelda, and Mario, and Smash Bros. AND of course, the aforementioned Metroid. Ironically, Microsoft's XBOX division, which practically lives off of third-party developers, depends solely on the Halo series. It always has, from day one. Nintendo, on the other hand, depends on the slue of first-party titles it has going for it. That means Mario, Zelda, Metroid, Mario Kart, blah blah blah and so on so forth. The Wii does have it's killer apps. Hell, I'd even say that Twilight Princess (which released on day one, alongside the Wii) could have been one of the Wii's many killer apps, if only it was not a direct port from the Gamecube version.