So as some one who hasn't played FF ever, this seems like a big loss for Square.
Cause from what I've seen and read, here's my understanding of what the situation of FF15 was:
1) Other director had his head up his ass, taking like 10 years to finish the game, resulting in the game going in dev hell (?)
2) Tabata came in and turned things around, actually managing to put out a good final product.
So if that's indeed true, that's a big loss for Square.
No idea what happened with Nomura and FF vs XIII/FFXV but he's pretty much done with KH3 which is coming out next month.
I think there's some backstage politics playing in the background that made the development for FFvsXIII/FFXV to go to hell for a couple of years.
Regardless, Tabata leaving is a big loss because it pretty much spelled the end for any future developments for FFXV at least in the light of new content and of course this means the FF Type series is pretty much dead since Tabata made the Type series.