Prime 2 was already superfluous, I hope they're not wasting their talent on another one of these games.
I'm pretty sure the franchise games sold Cubes, just not in the way you expect. They made the Cube appeal to fanboys (you know, the fanbase Nintendo isn't supposed to alienate) and therefore sold a lot to that demographic alone. Sure, they didn't buy it for a specific game but they bought it for the promise of Mario, Zelda, Metroid and other sequels. Look at what these fanboys expect from the Revolution, another Zelda, another Mario and another Metroid.