Lots contributes to this but I think above all the nostalgia for VII and the novelty of the remake is over. Now it's just hardcore fans invested and essentially a niche endevour.
Part 3 should be a condensed & much leaner considering this. I suspect it will still be Playstation console exclusive at launch but be day/date on PC. For Sony themselves, there is very little value in paying big bucks for something that won't really shift consoles and is wanning in popularity.
This game will sell less than XVI. Final Fantasy Itself kind of needs a soft reboot which feels both approachable (new naming convention)
& highly distinct enough to capture the attention of a new generation. Final Fantasy XVI imo looked too generic on a service level. Both the original FFVII and FFX were both landmark fantasy experiences when they released on PS1 and PS2 and kind of looked unlike anything else on each respective platform. Square need another highly original fantasy world and I hope a gameplay system which is aesthetically clean and tactical.