If it was a launch title, it would've ended up being one of GameCube's best-selling games, if not the best-selling game. GameCube would've had a completely solid launch window and probably would've sold a few more million than otherwise during the 2002-2003 period.
The bad part? It would've taken up one of Nintendo's core dev teams. Perhaps Pikmin might have never existed, or it would've been pushed to 2004 or something (although business-wise it's a smarter move). And really, one game (even though it's SMB) wouldn't reverse the GameCube's fortunes altogether. We're looking at a second-place in the console race scenario no matter what, and still I doubt GameCube would surpass N64 numbers.
Do you know what else could've really, really helped the GameCube reach N64 levels of non-suck? If Rare's talent exodus didn't happen, and if its owners didn't sell the company to Microsoft. That would've been awesome.
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