In a universe where Avatar 3D never existed, would the 2D version alone have sold as much? Remember, they would have also been able to deliver the movie earlier (I don't know how much, but making a 3D movie can't be free - at least not yet).
I'm reasonably certain that there are only a very small number of people who saw Avatar for the sole reason of it being 3D (in other words, if the 2D version was the only option, they would have skipped it).