Why on Earth would Square-Enix do this? I mean, seriously.
Making Versus into XV sends exactly one message to the world:
we are too incompetent to make a real Final Fantasy XV. We are so desperate that we have to take a standalone title that's already associated with a different numbered FF and rebrand it six years after its announcement because otherwise we'll never release another new numbered entry.
It's one thing to give it a new name to separate it from the stink of XIII, but trying to reposition this title as a new main-series title is only asking for trouble. Anyone who thinks this is a good plan should probably avoid weighing in on branding debates in the future.