I just don't want the final product to suffer because of the multi-part format. Once it all comes out, there's zero benefit to it being multi-part, so it better not be dragged down by it's release method.
Based on the interviews, it's seems like they chose the multi-part format because making it a single-part format is what would have made the final product suffer rather than the other way around.
As it is - splitting it up, means that they can start generate revenue before the game is finished, which they can then use to keep the project afloat and add more content as they go along.
There is no way SE would have floated Kitase and Co. the founds to sit on this game for 5+ years for example, after what happened with FFXV and KH3, which would mean that if they were to make this a single release they'd probably be looking at a total development time of 1-2 years at most - which would mean that they final game would suffer great content cuts as a result.
Literally, the only way to avoid this, at this point, is to make it into a series. At least, that's my impression.
This can backfire badly though if not enough people buy the first part, as that would suggest to the share-holders in SE that the project doesn't warrant extensions of time and funds, which will lead to content cuts again.
I just can't imagine who would even be starting with the 2nd or 3rd installment. I'm not saying that's not a valid question, or that it's not something they should be planning/programming for, but seriously - who would do that? But I'm sure they they will have to have it programmed so that the enemies are scaled even if they're not thinking that someone might pick it up after the first installment or two, because I'm sure not everyone is going to have Cloud at the same level at the end of the first game. It will have to have some room for level scaling/adjustment.
Although whoever isn't going to play the first installment of the game to hell might as well be dead to me
For all we know it might not actually even be possible to play the second or third installments without the previous ones on your hard-drive.
If they plan to design this using an expansion system, it's conceivable that the world will be stored on your hard-drive and that each new installment will simply add areas to your base-game, much like an MMOPRG.
Ex.
Part 1 : First Continent : Midgar to Junon continent with the story ending at the defeat of the first Jenova
Part 2 : Second continent : From Costa del Sol up until Tiny Bronco
Part 3 (and maybe 4) : The rest of the world (the Islands)