It would have been big, but not as big as you're giving it credit for, Demotruk. Remember - GCN didn't have the simple controller design that's a major throwback to the NES, and it didn't have motion controls.
It would have been assuredly less than 10m, but it also would have been the GCN's best-selling title and probably would have finished up in the 8-9m range.
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