Rebirth's sales missing the target is basically not news at this point, and given the circumstances around the game, hardly surprising. Exclusivity is a big issue, but its clearly just one of the several ones. At the end of the day its a crazy ass project and I doubt any other publisher would've approved of it - and I bet Kitase would get laughed out of square's offices if he proposed it today - it is what it is.
I'm more concerned for 16 failing to hit further milestones, because that's where the future of the brand lies, and it doesn't look pretty. I love 16, but it's becoming more and more obvious that it failed to capture the audience that showed up for 15. And with the huge gaps between mainline FF's, and the lack of consistent identity... well, it's starting to become a real problem. FF15 was only on playstation and xbox at launch, 3 years into the gen, same as 16 when it launched. And 15 hit 5m milestone in 3 days. PS/Xbox split was around 80/20. So yes, again, multiplatform would help but it certainly wouldn't account for everything, as 16 seemingly isn't anywhere near close to hitting 5m yet despite several discounts and 2 DLC releases and being nearly a year on market.