Bioshock Infinite will probably sell in at least the 2-3 million range. I can't tell you why it will, but it probably will as people seem to like this game series for some reason despite the major drop in quality with each title.
You start off with the great System Shock games then the still good but inferior Bioshock 1. We had the awful sequel to Bioshock and now Infinite which looks like a totally different game again, but still has the Bioshock name ... guess they thought Skyshock sounded stupid.
It all kind of reminds me of the Fable series ... start with great title and each title after gets worse and worse and then before you know it we will have on on rails kinect Bioshock game.