I think there is a clear slowdown in the popularity of the series, it's mainly coming from Japan, where FF is not what it once was. I think people do overlook the fact that, for the first FF in a new generation of consoles, FFXIII has preforms pretty poorly compared to the last two generations, where FF7 and FFX both set high marks for the generation, and the subsequent FF releases fell off a at steady percent. Given that FFXIII is just over 6 million sold, it's reasonable to assume, that if another non-MMO, non-spin off (Verses XIII is a spin-off) is released this generation it will fail to move 5 million units world wide, which would be the first time a non-MMO main series title has done that since FFVI.
I do look at Japan as the cause of this, since FFX each FF seires title has failed to move as many copies as the last, it's been trending down for nearly a decade now, it's not something new. The franchise will continue to be a major player though, through spin offs, re-releases, etc. It's not the FF I love anymore though, they have alienated a lot of fans with their changes to the core gameplay.
