Ya wanna know what was really unnecessary? Showing Riku and Mickey exposition dump in the early hours of the game, the absolutely strange "Aqua is like Sora" joke (which I still don't understand Riku's reaction to btw), Riku telling RR that "[he's] got [his] own" before facing AntiAqua down, every single moment in game wasted calling Sora an incompetent bumbling idiot. Showing Kairi and Axel just talk empty KH talk in a CGI forest, instead of showing them fighting and learning the rudimentary ways of the Keyblade.
These are terrible examples to pick as nearly everything here serves a necessary function to the story they were telling in KH3?
Showing Riku and Mickey exposition dump in the early hours of the game
This literally sets up one of the major themes of the game as well as Riku's arc. It happening early in the game is also key as it is directly paralleled to the arc Sora is going through. Sora ends Olympus with the understanding that in order to save the people he wants to save he'll need to find a special strength ("I'll find my strength the way you found yours, something to fight for, with all my heart"). The scene with Riku is the answer to the question posed in Sora's arc. Mickey tells him that that special strength comes from caring so much for someone that all else fades away, that there is no room for fear or doubt. Sora's arc is about finding that special strength and what it means and he learns what it looks like and what it means to care for someone like that through the game through the Disney worlds. Riku has already finished that part of the arc (it's what he learned in DDD) and his arc is now about what it means to actually use that strength. This early scene is setting up the key thematic throughline of the game, as well as the set up for
Riku's actions in the Keyblade Graveyard. Beyond that, on a more surface level, these early scenes are establishing the complications in the search for Aqua and her status, the necessity of the power of waking, and setting up that whole aspect of the game. Hardly unnecessary.
the absolutely strange "Aqua is like Sora" joke (which I still don't understand Riku's reaction to btw)
It's not that difficult to grasp. Riku is taken aback at the suggestion that this serious Keyblade Master he's only heard glowing things about could be just like his lovable dumbass reckless friend Sora who throws himself into dangerous situations without a thought and doesn't take things seriously. This is why he calms down when Mickey clarifies that Aqua is strong like Sora, not that she acts like him. As for what function this is serving, aside from providing a moment of levity in a previously heavy-handed serious scene, it's meant to both put some ease on the pressure the characters are under (while Aqua does need to be saved, throwing themselves recklessly into the depths of darkness without a plan isn't smart, knowing that Aqua is strong like Sora and that she'll be okay while they figure something out) it also is used to draw a clear connection and parallel between Aqua and Sora. This helps set up Sora saving her later in the game, while also f
oreshadowing that Sora will befall a similar fate to Aqua in the end of the game.
Riku telling RR that "[he's] got [his] own" before facing AntiAqua down
I feel like you misunderstood this? Riku isn't telling Repliku that at all. He's replying to Aqua based on something Aqua had just said. Largely though, this, again, is not unnecessary, it serves a function. It's calling back to Riku and his character arc, primarily. Throughout KH3 Repliku represents Riku's Past Self (and by extension, the mistakes he made before) and how he's grown from it and treats his past self with compassion. Obviously Repliku is not Riku's Past Self exactly, but it's what he represents for Riku. So this scene is calling attention to that. Riku explicitly references his past sorrow and hurt and then the character that symbolizes that past appears (wielding the Way to Dawn no less) and helps him regain his strength to face Aqua. It's also serving the necessary function of reminding the audience and players of Repliku and of Riku connecting with him the last time he was in the Realm of Darkness. Most key elements in stories follow an arc of Set-up -> Reminder -> Pay Off. The early scene with Riku and Mickey in the RoD where Riku crosses paths with Repliku is the Set-Up. This scene, toward the start of Act 3, is the Reminder. With the pay-off happening in the climax. So, yeah, it is necessary and performs multiple functions in the story.
every single moment in game wasted calling Sora an incompetent bumbling idiot
Aside from providing moments of levity and characterization (which are far from being unnecessary?) this is also extremely clear set-up for Sora's lack of self-worth and why he falters in the Keyblade Graveyard ("Without them... I'm worthless."). Like this is so incredibly obviously serving a purpose in the story?
Showing Kairi and Axel just talk empty KH talk in a CGI forest, instead of showing them fighting and learning the rudimentary ways of the Keyblade.
Scenes of them training and learning rudimentary Keyblade maneuvers is like the definition of an unnecessary scene. Would it have been nice to see? Sure. But it wouldn't have added much to the story. If anything, it might actually make the story feel worse (having increased emphasis on Lea and Kairi's actual training would have just made them whiffing it due to being new to the Keyblade in the final act be even more pronounced as it's putting way more investment into it).Also the claim that the Kairi and Axel scenes are just empty talking and not necessary is wild to me. Honestly, it's the remark that pushed me to bother replying because I just found it egregious. Both scenes are very clearly performing multiple functions for the story.
The first scene is clearly setting up Kairi and Axel's storylines for the game with them learning the Keyblade in order to be able to wield it in the final battle. That much is obvious. It's also establishing their relationship. It addresses their past relationship and how things have started to be mended but isn't quite there yet. They're distant with each other. They sit far apart. Their conversation is wooden and awkward. This is all characterization and set-up, and necessary. In addition to that, we also have Kairi's letter to Sora which sets-up their relationship for the game, as well as setting up Sora's larger arc and his character. It also foreshadows Sora cracking under the pressure of trying to save everyone. It also reveals Kairi's character -- she no longer wants to just sit and wait around, she wants to do what she can to help. On Axel's side, this is literally the scene that sets-up Xion in the game and reminds players of her and how she is part of his goal and motivation even if he can't remember her.
Stories need set-up in order to have pay-off. That applies both on a larger series framework, but also in individual entries of stories. You can't have Xion show up in the Keyblade Graveyard at the end of the game, without having her being set-up earlier in the game. Some of these scenes may seem unnecessary to a diehard fan who, for example, knows that Xion needs to be saved or whatever from a past game, but that doesn't change the fact that they are necessary on a fundamental story level to be included. That's how stories work.
Anyway, so far, a lot of necessary stuff in this first scene. Let's jump to the second scene of "empty talk". Here their training is almost finished. Immediately we can see the change that has happened in their relationship. They talk more easily with each other. They comment on each other. Kairi teases Axel like three times showing a comfortable familiarity with each other (while also showing Kairi's personality). Axel's refusal to change his clothes plays into his larger motivations (though most people don't seem to pick up on this one, but he believes he'll have a better chance of saving Roxas and Xion if he looks like how they remember him (this is the same reason he asks to be called Axel again) as he says "This is how you pick me out of a crowd. I make myself easy to remember")). Now, Kairi and Axel sit right next to each other. An immediate visual cue of how they've grown closer and more comfortable. With their training being finished, they are both dealing with anxiety about what it means to go back into the "Real world" and face these things (set-up for how things go in the final act!). Kairi dives into her motivation more,
specifically about wanting to save Naminé. She gives what is honestly a character (and game) high-point for her, where she delivers
a beautiful speech about Naminé and articulates wonderfully why these people deserve to be saved, why they deserve a second chance. Previously, these characters being saved had only been described in utilitarian ways by Yen Sid ("they have more to achieve") and them being needed to fight Xehanort. Here, Kairi reveals the humanity of the situation and why it is fundamentally important for them to be saved, not just to fight in some war, but because they deserve more than that. It's a beautiful scene, written and delivered well and an important point to underscore right before the break into Act Three which focuses on saving these characters. To call it empty talk is honestly insulting.
Axel then talks about Ven, and having met him as a kid. This serves a larger point of reminding the player of their connection, but it also is getting into Axel's character. Axel has always been concerned about people remembering him. He anxiously asks Kairi if she thinks Ven would even remember him after everything. This is obvious characterization for Axel, but it's also a callback to Ven and Axel's first meeting where Lea explains why he's so concerned with people remembering him ("I want everyone I meet to remember me. Inside people's memories, I can live forever"). He admits that now that they're going back into the real world and having to face all of these things, he's "worried about everything." Kairi replies, "Well, you don't have to worry alone anymore, Axel." A beautiful line that honestly stuck with me still. It captures just how much they've grown close to each other. In many ways it echoes their first meeting back in KH2, where Axel tries to trick Kairi into coming with him by pointing out how they both miss someone they care about and sarcastically says it's like they're already friends, to which Kairi rebuffs it. Here, they are both openly commiserating about missing people they care about and how important it is to save them, and have become actual genuine friends, no longer needing to worry alone. It's lovely, and accomplishes far more than if these scene were just replaced with scenes of them play-sparring or getting Keyblade 101 from Merlin.
Both scenes are accomplishing a lot, on story/plot levels, character levels, development levels, thematic levels, etc. They're far from empty KH talk.
None of this is really unnecessary.