Sorry, I think I explained myself badly. After the original Nier, the story was finished. That doesn't mean that no more games were possible (indeed, Automata), but the story was a complete package. We knew the main mysteries of the world, we knew the consequences of what we had done, and nothing else was strictly necessary. Then Automata came out, which was good. And again, that game is a complete story, and nothing else is necessary. It was a complete game.
It wasn't really suffering to be a Nier fan after Nier the same way it's not suffering to be a FFT fan, or a Chrono Trigger fan. I would love more games in those universes, but if I never get them, those are complete universes.
KH3 isn't like this. KH2 was! So was Days. But KH3 isn't complete. There's obviously a throughline that hasn't finished, and it's not some ancillary thing. So (for me at least) it's a far worse feeling being a KH fan than being a Nier fan, because KH3, CoM, MoM, BBS (kinda), UX, are not complete. Which is not a problem for something like CoM since a game came out shortly after, but it is a problem for something like KH3.
This is all my personal feelings on the matter. People might feel differently.