You answered your own question in the topic title. The fact that it took so long to make FFXV is possibly the reason it all went downhill. Ambition is one thing, but there was no way they'd be able to keep their own hype and thoughts consistent for so long, as new things come up to conflict with the original ideas, and then to change hands in the middle of it all as well.
Things that are fresh become stale the longer out it goes, and it's possible that some of the original ideas that first came out became too warped and changed over the long wait, and I'm sure some of their workers got exhausted rehashing the same ideas and concepts, that over time they probably lost interest.
This why I think it's great to have ambition, but pair it with a realistic timeframe and realistic goals. By X date, decide on something and go with it, otherwise you're stuck in a perpetual cycle of concepts, ideas, prototypes, feedback, and potential delays.