GameCube could've sold 40-50 million units, Nintendo just mishandled the console entirely. It should've launched in fall 2000 for starters.
If they had launched a year earlier by moving some late gen N64 projects over (Zelda: Majora's Mask, Perfect Dark, Conker's BFD ... these games barely ran on the N64 anyway) they would have done reasonably well. Not enough to deter Sony from winning the generation but enough probably to close Microsoft out of the market (1 year too late and XBox is suddenly a third wheel console).
N64 could've been transitioned over to being more a Pokemon/kids console by 2000 or so, hell they did release that Pikachu N64.
I think $199.99 as a price point was a mistake too. $249.99 with double the RAM would've been a better play IMO, really would've increased the gap between PS2 and GCN multiplats and Nintendo still would be cheaper. Mini-DVDs was stupid too, use full size discs, if people want to watch DVDs, they can buy a $30 Nintendo Remote (which cleverly would off-load the DVD licensing fees away from the system).
