Final Fantasy XV ended up as an amazing entry that's more than worthy of being an FF title. It suffered a slow start due to being rushed out of development hell (this is the same game that was trapped in its VS XIII hell) but to somehow tout that as indicative of being a failure is just plain ignorance. It's successful and loved for a reason.
As for KH3, that's an entirely different team and game series with its own unique curcumstances. It's so divorced from FF now that comparing it or trying to gleam any indication from its status is irrelevant. Those issues and development trials don't exist for FF. But yeah, that's successful too.
And DQ XI sold like gangbusters as well. So I'm not sure *what* point you're trying to make.
I loved Final Fantasy XV, too. It started out slow for me, but once I got into it I was really into it and even platinumed it. That turtle was a fun fight!
As for the whole locking us out of areas with each installment discussion, I would be okay with it only if it were like it already is on the original game, as was already suggested here. When you have the tiny bronco you can only go to certain areas, obviously, and that sort of thing. I really, really hope that they don’t lock us out of the previous installment every time a new installment releases. That’s where they need to find a clever way to do this “episodic” thing.
But we all know that if it is in three parts, it won’t be broken up the way the original game was. Not if they are going for a “full game” on each new release. If the third game equaled the third disc, there would hardly be anything left to do. All of these reasons are why I still believe two parts is the most logical way to go about this, but they probably will do at least three for the cash grab.