Simple economic answer: hell no XV won't be the last numbered entry. I highly doubt there ever will be a "last numbered entry" unless Squeenix seriously mismanages some money and has to shut down. It's one of the most popular franchises in existence and their flagship series, and no matter how many spinoffs, non-traditional sequels, and HD rereleases they can pump out, eventually there will be demand for an FFXVI, and an FFXVII, and so on and so forth.

As for what they'll do next (other than the obvious VII remake), I imagine a few spinoffs to XV, maybe an XV-2 if they feel like they could possibly squeeze a direct sequel out of a fairly definitive ending. They were already devoted to making XV a massive multi-media juggernaut with far too many tie-ins, and XIII got far worse reception across the board than XV before getting two sequels, so why stop now?