Well, I think all the mario games could've lead to sunshine's relatively low sales, that's an interesting point.
...but, more likely, I think, is that sunshine's late (i.e., not at launch) release date, combined with the game's somewhat underwhelming content (don't get me wrong, jumping around in that thing is INCREDIBLE....but you have to admit the game is no Super Mario 64), combined with a surprising number of apparently competent 3d platformers this gen (Jak and Daxter, etc), are what lead to the game's disappointing (though still quite good) numbers.
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Originally posted by: Ian Sane
Mario spinoffs sell games to an existing userbase but they don't sell consoles.
Regardless, they still sell well and make money.
Though I do agree with you that it's troubling that Mario (and Zelda, and Metroid) games used to be for everyone, but now it seems they're only really fro the initiated. Though I'm not sure spin-off games are necessarily to blame for that, so much as the advent of ultra-violent, ultra-freeranging, ultra-epic games that have eclipsed Nintendo games in the eyes of the average gamer.
What Nintendo needs at this point is something to draw gamers back. Something radical that will make gaming more fun and yet more intuitive. Something like, I don't know, a freehand motion controller.