That thought came to my mind while reading this article: http://www.computerandvideogames.com/327700/final-fantasy-vii-remake-would-take-too-long-square/
Especially the last statement:
Graphically they weren't as advanced as they are now, but there were lots of towns and worlds and cities and whatever. So if we were to recreate the same kind of game - sequel or not - with the same volume, but give it a much higher level of graphical quality, it would us take three times, four times, even ten times longer to make such a game. So making a sequel for an old game would be a lot more challenging.
Is this actually why Versus takes so long?
Because the game seems to feature some of those aspects. A big, more open world, live combat without switching to a battle screen, airships, a car to move around places. Sounds like a lot of content, a lot more than XIII and XIII-2 had. And considering the graphical quality, this game will be huge. My XIII-2 savegame was already 23MB big, which beat my biggest savefile until then (Skyrim with 11 MB).
So, maybe the first XIII games were just warm-ups to keep the crowd in line? Think about it, if we were to wait five or six years until the first FF comes out on a next-gen console, the expectations would be overwhelming. It could be a tactical move to release two "minor" games until the big one comes out. Their production would be cheaper, but they would still sell well.
Just a theory tho.