It's not like Twilight Princess sold badly, at 7.1m it's just 500k under Ocarina of Time. Including the GC version at 1.6m it's a total of 8.7m. I doubt it would have sold any better if they called it: "The Legend of Zelda: Shadow of the Wolf" or something instead. Especially since it seems Zelda sales aren't that heavily dependant on install-base. Most average around 5m regardless of platform, with only the two that released extremely late in their console's cycle clearly under.
As for 10m sellers I hoped A Link Between Worlds would have finally be the one that had a good chance. Not because it's good enough to deserve it, but ater New Super Mario Bros.' success, I hoped having a retro-style Zelda would have a similar effect. Even when VGC undertracks this game by like a million units, it didn't appear to be that people were really waiting for an old-school Zelda after-all. Especially comparing it to the sales of Ocarina 3D, which are nearing 4m, it seems clear what people want. Hopefully Zelda U can deliver.
To sell 10m, they probably need a Ocarina/Twilight Princess-style game that boasts modern gameplay elements with great graphics that aren't held back in any department in big worlds and surround it with tremendous hype to have a chance. On a userbase that's big enough, at least over 30m at the end of that system's life, and be released during that system's peak. They would also need a bit of luck. I hope it happens someday.







