Halo 3 was one of the largest entertainment launch in history. Not just games, ENTERTAINMENT, period. As in, it was a launch that rivaled that of TV Show, Movie, and Video Games alike. An estimated 3 million units of the game were sold in it's launch period alone. To put that in perspective, Halo 3 since it's launch in September, is already close to selling more than Super Smash Bros. Melee's total sales, from 6+ years being on shelves.
I don't even own a Xbox 360, and the most of Halo I've ever played was Halo 1 multiplayer with friends, and even I know that there's no way Brawl will come close to topping what Halo 3 has set. In the opinions of this community will Brawl be an entire superior and more enjoyable game? Sure. But to even conceive that it will sell more? Not a chance.