I think one of the reasons that the first new iteration of a franchise each gen sells better than later iterations during that same gen is because of the major leap in graphics from the previous gen.
That was worded horribly, so I'll use an example: Gran Turismo.
GT3 sold upwards of 14 mil. Why? Because it looked absolutely amazing compared to GT2. Gt4 only managed to sell ~8.5 mil. because, while it was still an excellent game, it didn't have quite the "WOW" factor that GT3 had. I think something similar may have happened with MGS as well, and I have a feeling that both GT5 and MGS4 will sell better than their immediate predecessors (GT4 & MGS3). Something similar will most likely happen with games like Tekken 6 and FFXIII as well.







