Halo 5 probably won't sell as many for a few reasons. First, when it releases it's unlikely that the Xbox One will have sold as many units as the Xbox 360 had 2 years after its launch which is when Halo 3 released. So that alone will make for a hurdle to the sales. Second, the FPS space is going to be much tougher this time than it was when Halo 3 launched. Hell it will probably be much tougher than it was for all of the last generation. With Call of Duty still being a big deal, Battlefield making a name for itself on consoles, Titanfall releasing earlier in the year, and Destiny launching a couple months prior (assuming a Holiday 2014 release) the game is going to have its hands full trying to pull people away.
If 343i doesn't try to really do much new with the game and people feel that the franchise's gameplay has become stagnant that will also affect sales. Of course that's just a "what if" reason. Something that I have noticed though is just a growing lack of interest in the universe of Halo. It's still big. There should be no doubt there. I just don't get the same sense of excitement from people over it like when Halo 2 or Halo 3 came out. By the time Halo 4 came out the hysteria surrounding the franchise was not quite what it was about 5 years before. It will likely still sell 5 million copies or more when it's all said and done. I just wouldn't expect another 9 million or more sales unless Halo 5 blows people away.







