Though I see many people arguing about how the game would sell more multiplatform (no shit), that could be altered depending under what circumstances the game is released. If the game, let's say, was released as a PS4 exclusive around the time it launches, the game could actually see higher sales than if it was multiplatform.
We often see that games at the beginning of a consoles life selling far more than we would have expected from a multi release, since the game is of high quality and has little competition from other games.
Just look at Fall of Man. It was a completely new IP and, thanks to being released during the PS3's launch era, went on to sell over 4 million copies. Final Fantasy could really do with that kind of boost at the moment.
I could easily believe Sony and SE coming to some form of agreement that locks down exclusivity, with SE gaining the benefits of being a launch title and Sony agreeing to promote the game in some form.
In the end we can only guess until E3, so we will see ^^
