I just don't think the public really generalizes between "2D Mario" and "3D Mario" in the way that most gamers do. Yes, the 2D games are perhaps easier to grasp for some people, but I don't think people really go "well, I liked the last Mario game, but this one is 3D, so no thanks". Some people, maybe, but certainly not everyone. Obviously, New Super Mario Bros. Wii has more of a simplistic approach and throwback that made it catch it on, but I don't think it's impossible for that appeal to translate to Galaxy 2.
I'm not saying that Galaxy 2 is going to sell 3 million copies in Japan, but I think the fact that Mario has been firmly back in the public consciousness because of the recent game will definitely give a boost to Galaxy 2's sales. I could easily see it surpassing the first one for that reason.