Why I think XV won't disappoint: because Square has obviously learned from their mistakes. Final Fantasy XIV is SOOOO much better in ARR than 1.0, it's crazy to think that 1.0 even existed. And while I was one of the people who enjoyed XIII (its combat system is amazing once you finish the 25 hour tutorial), it did have definite flaws, and XIII-2 not only dealt with almost every one of those flaws, but gave the game a Chrono Trigger feel! XIII-2 is the closest they've come to a Chrono Trigger 3. If they can do that to FFXIII, and FFXIV, I could see Square fixing any bad game.
I have a feeling the main reason FFXV has been taking this long because the Square that learned from their mistakes in other games, saw that they were making similar mistakes in this one, and decided they really needed to fix it before doing anything else with it.