My argument was never whether Leon and the others should have been in KH3 or not. My argument was whether they were important to the main plot -- which I do not agree with at all. Maybe it's because I see it that way that their absence in KH3 doesn't really affect me. I did think it was strange that we didn't see them at all and I was looking forward to what contribution they would make in KH3 (I thought it would be something to do with Cloud/Sephiroth).
And my argument is that you're drawing arbitrary conclusions based on personal feeling. If Nomura had written the Restoration Committee to be important to the main plot, they would have been important to the main plot. Leon could have just as easily been pushing those buttons and making those phone calls as Ienzo-- which is not to say that's the role I would have liked to see him play, but if that's the minimum threshold in KH nowadays for "main plot relevance," it's not exactly a high bar to clear.
I think the issue comes down to Radiant Garden itself. I don't think Nomura ever planned for Radiant Garden to become the home world of so many characters, specifically the Final Fantasy and Organization members/Ansem. Because the Final Fantasy characters never got fleshed out more on what their original purpose was in Radiant Garden, we have no idea what their roles were.
We know that they lived in Radiant Garden, and it was their home world, and it was violently stolen away from them. Which is literally as much as we know about Sora, Riku, Kairi, and their relationship to Destiny Islands. Moreover, you're minimizing their motives as characters: Leon, Aerith, Yuffie and Cid don't just take up the role of restoring Radiant Garden; their whole mission is to act as its guardians and oversee its continued prosperity. Cid tried setting up an entire automated defense system to this end. To argue that they just wouldn't care or wouldn't notice or wouldn't be involved in who is coming and going from the castle is in direct contradiction of what we actually know about their position within the world. You may not "feel" that it's important, but the actual internal logic and continuity of the series begs to differ.
All we knew was that they were restoring it and in KH3 it looks restored completely. We also know that Ansem The Wise, Apprentice Xehanort, Dilan, Aeleus, Even, Ienzo, Lea, and Isa are from Radiant Garden. Unlike the Final Fantasy group, we actually know more about what those characters actually did in Radiant Garden and interacted with them beyond just the exposition the Final Fantasy group offered.
In what way do we know anything more about the Apprentices than we do about the FF crew? All we know about them is that they were on RG, got their hearts stolen, became Nobodies which made them ~evil~, then they got restored and some of them stopped being ~evil~. That's the entire knowledge base we have for Ienzo's arc: we know nothing about his specific relationship with Ansem the Wise except what is alluded to in a couple of brief scenes, and even less about his relationship with any of the other Apprentices. The Organization members who are actually fleshed out as characters and play major roles throughout the series aren't even Ansem's Apprentices; Lexaeus has literally one or two scenes in CoM and one line as a guard in BBS. Xaldin is actually and undeniably a horrible person for the sake of being horrible in KH2, then inexplicably flips without even a mention of his past misdeeds or what might have motivated them in KH3. Vexen is purely functional plot fodder; again, we know next to nothing about his specific qualities of character, what makes him tick, what his relationships with his colleagues is or was like. Just that he conveniently "wants to atone" so the good guys can get their Replicas-- which, ironically, makes Ienzo even less useful or unique.
Compare that with the Restoration Committee, which come from the same basic drawing board but have actual arcs beginning with KH1, where they enjoy an explicit connection with Sora and the journey he's on-- providing critical information/assistance, regularly acting as a springboard from which he grows and develops over the course of his adventure, bookending his quest by returning to reclaim Hollow Bastion and affirming his efforts with a final salutation that carries such thematic weight it becomes part of the bedrock of the next two games. Then in KH2 we see them all working as a team to defend their world and make it livable and safe again, and they once again provide Sora with a home away from home-- something no other group of characters could have done, and something which is sorely missing in KH3-- before helping him to solve one of the core mysteries of the game. They may not, themselves, be at the center of that mystery (or any of the mysteries in KH), but that's hardly the point: on the contrary, their role is invaluable because they have both a sufficient level of connection to and distance from the "main plot" to perform the essential task of supporting the weight of the story material without overshadowing or becoming entangled in it. The fact that we don't need to know more about them than we already do is the beauty of it: what they inherently provide from the perspective of the player-as-Sora is a sense of emotional trajectory, from KH1 through to this newest (and ostensibly most harrowing) journey. They are an essential link in the chain of Sora's history as a character. That's what makes them matter.
I think it came down to which characters we needed to see at Radiant Garden in KH3. We didn't need to see Leon and the others (as much as we all would have liked to see them). We didn't need to see Aeleus or Dilan, but I think Nomura put their silent selves in there so that there wouldn't be any confusion on what happened to them after they became whole.
Except that it makes no sense whatsoever that Leon and the gang aren't present in KH3 given the framing and context of the story as it has been presented over multiple installments. As a result you can't explain their absence, you can only insist that you don't care about it.
My question is this: What would Leon and the others have done in KH3?
Well considering they were living with Merlin and Merlin has a certain important task which involves preparing certain important people to partake in a certain important war-- gosh, I can't imagine a scenario in which these characters had any relevance or contributed in any way shape or form to the events of KH3.
In fact if they were involved in this subplot it would actually help explain why Merlin just dips from the essential duty he was supposed to be performing mid-game to go get his PSL fix.
They know nothing about Master Xehanort, the "Real Organization", Roxas, Aqua, Terra, Ven, the X-Blade, true Kingdom Hearts, the keyblade war, and everything else that's going on in the plot.
They're living with Merlin, who knows all of this.
It would be a waste of time to see them get enlightened about it because we already know about it.
"Hey, Sora."
"Leon! It's been ages!"
"Merlin filled us in."
Job done.
I mean it would have been a nice cameo to have them be absent all during the game and when Yen Sid showed up at the Keyblade Graveyard, Leon and the others were with him and they just yell out something about "Sorry we took so long, we were getting caught up on the details" then a little fan service fighting cutscene, then the plot moves on. But I can't realistically see them doing anything more than that.
Again, this is all rooted in an arbitrary series of presuppositions on your part. None of your arguments hold together on the basis of what is actually evidenced within the series itself, or even this game taken in isolation. It's not like the KH3 we got is in any way superior for its lack of FF characters: it's not like we spent more time learning to understand the characters who are in it and getting grand, meaningful arcs for them because Nomura took all the time he would have spent on Leon, Aerith, Yuffie and Cid having actual conversations with Sora and put it to more effective use elsewhere. It's not like deep friendships are forged or rekindled between Ienzo and Sora, or Vexen and Demyx, or (lmao) Dilan and Aeleus: these people are coworkers and opportunistic allies, at best, and when they don't have plot jargon to espouse they aren't in frame. What exactly is the contention here. That we couldn't have gotten that one (1) scene between Ienzo and AtW if the FF crew had been around? Or that we wouldn't have gotten that one (1) scene of like 1/3 of the main cast throwing past plot points at a dart board in Yen Sid's closet together? Yeah. Remember that great Namine arc we got because Nomura decided to cut out Leon and the gang? Oh wait. Remember that great Kairi arc we got because Nomura decided to cut out Leon and the gang? Oh wait. Remember that great interaction between Ven and Roxas, or Sora and Roxas, or Kairi and Xion, or Kairi and Namine, or Aqua and Eraqus, or Ven and Eraqus, or Xehanort and anyone who isn't Sora? Gawrsh, I know I sure don't.