This is just what I know best to my knowledge so some of it may be off, but here goes.  It is actually quite simple I think.  They stated that FFXIII Versus will be set in the same world as FFXIII and delve into more of the mythology of the Fal'Cie and what not.  Well, there has never been two MAIN ENTRY Final Fantasy games that were set in the same world.  For this reason, Versus cannot be FFXV because that would break the trend.  Now granted this rule is not set in stone, but I don't think Square-Enix would break the tradition.  I know some out there will say FF Tactics and FF XII were both set in Ivalice, but I don't consider FF Tactics a main entry title, although it was still pretty damn good.   What do you guys think?

It's supposed to be in the "same universe" but a different world with totally different characters and a completely unrelated story. So I don't think this necessarily means anything.

The only link I think they revealed (so far) is Etro.

Rename her in FF XV = problem solved.

I think the fact that the gameplay is a lot different than any other main FF game before is a better evidence to this, but who knows.

I think they said that Versus was going to explain the whole crystal thing going on with the Fal'Cie and the l'Cie but like you said, they could just change that and it no longer has to be the case.

I don't think they are going to make Versus into FFXV because they would want to start making FFXV as a fresh game rather than just changing a game they are already half way through. Especially since that game has been revealed and has its own trailers and stuff.

Given the negative baggage attached to FFXIII, renaming it FFXV would probably be a good thing.

I don't really care what they name it as long as they just release the thing.