I think it's safe to say that it was in SE's best interest to get FFXV out from under the XIII umbrella. People are still mad about the first two games, and while the third one looks better, first impressions never truly go away. Besides, at this point it has diverged to from the original XIII to the point where you can no longer tell where they were related in the first place. I think giving it the new title will not only make more people willing to give it a chance, but allow the game to stand out on its own much more clearly.