Even if Aqua and Ven were saved earlier, it wouldn't really lead to a whole bunch of new interaction or anything. It's not like they're going to replace Donald and Goofy in your party and go to Disney worlds with you. (And once Ven was gotten back, why would the Disney plots exist at all? They have the guardians, they have the power of waking. Why would Xehanort wait?)
I think it's easy to forget that the character trio build up was for Donald and Goofy and Sora to journey together, that that combo is part of what makes a numbered title a numbered title, and that it hadn't really been a focus for like six games. (It's not a coincidence 0.2 ends with them getting back together as a team and Yen Sid literally saying "I'd have it no other way".) That was clearly the focus for KH3, it was their journey, and I think it delivered in that sense. It's why they are the ones who get the triumphant moment in the final battle, they were the ones that were with you, on this journey, from beginning to end, etc.
I also think we have to consider the fact that, unlike movies, games are meant to be played. The player is meant to be the focus and the developers need to juggle that. Nomura's quote about his difficulty with the KG makes it pretty clear that they were aware of the desires of fans, of the fact that each character has things they want to accomplish and that he wanted to make it so they each got to deal with their stuff themselves, but by the nature of it being a game, that the player character is Sora, that it wasn't feasible for them to make any more playable characters (and would also be awkward for endgame fights to be heavily based on a character you cannot customize or have relation to all the powers, skills, equipment you gained throughout the game), that it was something they struggled with, that they went back and forth on and rewrote again and again, but that they ultimately chose to focus on the player's actions and to do what they could to keep the pace of gameplay quick in that final segment. When you can't make all of these key characters playable, you end up having to limit yourself to everything happening in cutscenes, without any player input. I don't think that would have gone over very well either (I can just picture the threads of "KH3 is trash. A battle against 13 and you only get to actually play 6 of them?!" etc). Honestly, they were in a damned if you do, damned if you don't position when having to deal with the realities and limitations of game development.
I don't know. I always maintained for years that, for character interactions/development etc, the title to look forward to was KH4, not KH3. I knew that the game would be too focused on the plot, and on saving these people (and wanting these savings to happen in the climax) that there wouldn't be time for the interactions that people crave and want, so I wasn't too disappointed in that sense. I knew it would happen. It's a numbered title, that's kind of their M.O.s. I think part of the disappointment comes from everyone knowing they would be saved, so naturally the thing they wanted was The After, but KH3 isn't the end. There is more to come.
(Though, for me the game was full of great character interactions and character moments, it just wasn't really about the Guardians. But the interactions between Disney characters and the Org? The interactions between the Org members themselves (the Demyx/Vexen scene is amazing, Demyx/Zexion charming, Demyx/Larxene hilarious, Larxene's "Ugh. Xemnas." when he shows up, haha, Luxord/Vexen scene managing to be true to their characters, reveal past history and details, raise doubts, etc) like I loved all of these scenes, I felt like the Org members finally got to actually show their personalities and play off each other and we got unique matchups that hadn't been seen in the series yet, Vexen's surprising atonement arc, HPO getting focus and interacting in the main plot, the way Sora and co interact with the Disney chars (I'm especially a big fan of BH6's interactions and Toy Box's and Rapunzel's), Axel and Kairi's friendship, Riku and Repliku, Jiminy gets some nice moments with the gang and Sora, Sora/Pooh, Sora/Nami, Sora/Ienzo, Sora/HPO, Sora/Merlin). Heck, even within the Guardians you still have a decent amount of moments, and some that people have been wanting for a long time (Lea/Ven, Lea/Saix, Aqua/Kairi, Aqua/Sora/Riku, Ven/Aqua, Vanitas/Ven, Sora/Xion, DiZ/Apprentices etc etc)). Idk, for me there is still a lot of good here.
I understand why Nomura made the choices he made with the KG. I think he pared it down a little too much personally, but found that he still managed to hit the big emotional beats right for me, but that also seems to be something he's aware of as well and intends to address it in the DLC (personally i'm hoping that, if the battle structure can't be changed, that they will at least add special team attacks for each guardian team up in the KG, and I'm hoping for a few extra scenes of character moments). For some this is "too little, too late" which I can understand. For some, having to wait for those interactions for future titles isn't good enough. I get it, but I don't really know what more there is to say about it at this point. Is the next five years+ of this fandom and community just going to continue to be the same arguments, the same threads, the same list of disappointments, the same criticism, the same insults at the devs, and so and so on? Like at a certain point accepting and moving on is a necessity. Finding a constructive path forward is needed. Lest we poison our own well.
Anyway, that's where I'm at.