The last time FF had a major role that was integrated into mainline KH was KH2, which came out in 2006 globally.. 14 years ago.
Since then we have gotten
358/2 Days
BBS
Coded
DDD
All of which had no major inclusion of FF characters, and looking at DDD they really should have been there for the RG scenes.
Story wise, it is a flop that they fell off the face of the earth, but that was evident back in 2012 when DDD was released. Also I kind of feel like the significance of FF characters is being a bit over emphasized, when the only games they had a real presence in were KH and KH2.. like that's literally it. (Outside of rehashes like CoM or Coded, but those aren't true new interactions, just reiterations) Yes BBS had Zack (which I absolutely love and wish we got to see more of) but I am not surprised that never went anywhere else seeing as how that prequel takes place ten years in the past.
So basically, I don't agree that the team was putting a ginormous emphasis on FF. The emphasis is equivalent to a Disney world like Agrabah or Wonderland. The main difference being that FF characters more directly affect the main story, but once again, that hasn't been the case since 2006.
And the larger the KH universe has gotten, the less space there has been for FF. It is sad to see it go, but now with all the new characters being introduced through UX and the Secret Episode/Verum Rex, its even less likely for them to get the spotlight here.
I think the fan expectation was mainly there because it was KH3 and FF had a big role in one and two, but in reality that was a long time ago, and that has not been a consistent trend in any of the other games.
And not one of the games you listed where console games, and we got Twewy in a substantial role in DDD. Wouldn't really make sense to have Leon be in a sleeping world. As for Days, it's the most KH title ever, if that game could get rid of disney worlds it probably would. I still think FF characters could have shown up in the background perhaps, I think seifer only got a mention, but a DS game can only go so far.
The point is KH3 as a home console game was supposed to be a return to "form", with "form" being KH1 and KH2. Sure the FF has been diminishing in recent titles, but the size of the game should be taken into account too. We already know that BBS was gonna include young restoration committee but the psp could only allow for so much, or at least that's what we were told.
I don't think anyone here expected them to be as centre stage as in KH2, but we at least thought there'd be a short radiant garden segment. There's enough subplots in that world for it to be justified. I think it's pretty revisionist to say it was obvious all along they'd be cut, when the most recent title had Five square enix characters in a not insignificant role.
I'm sure the line of thinking was: "if Twewy can get a look in hhen we're sure to get Cloud, maybe even Lightning or Noctis"
You can't deny it was a large enough part of the entire KH3 hype cycle; discussing which characters would make it in (and I say would, because it was almost unthinkable for them to not be in)
As for their importance, what exactly do you mean by that? Is importance how many plot threads a character can rattle off in quick succession? Or is it a character who brings life to a scene with a fun personality? In terms of KH both are pretty necessary, if I could though I'd totally do away with the former.
They may not have main plot ties any longer, it's debatable if they even did in KH2, but they are pretty entertaining, moreso than 90% of the current cast, and I feel like that's pretty important for a game that was made for entertaining. Until they are replaced with something equal or better in entertainment value, I would never discount their importance.