You're basically reaffirming the fact that you prefer a day-to-day type friendship on screen rather than an implied or pre-set one. Sora, Donald, and Goofy are the same as Roxas, Axel, and Xion. We play through their friendship developing almost every day rather than exploring a friendship that has already been established prior to the game start such as SRK or TAV.
The point of KH1 wasn't to establish a powerful unbreakable friendship in the very beginning. It was meant to establish that those characters are close friends, but it had to be tested. Sora, Kairi, and Riku passed the test in the end (after x amount of games) and were able to fix their mistakes (too much competition, possibly liking the same girl, Riku selfishly sacrificing many people just so he could have an adventure, etc). Aqua, Terra, and Ven did not pass the test, or rather, they have not gotten the same opportunities as Sora, Kairi, and Riku have. Yet.
I think they did plenty enough to establish their friendship (as I already explained, though I forgot the mention the significance of the charms and the paopu fruit). Aqua's charms, their awkward laughing and the spot where they decide to chill often, training together, growing up together, implying they are a family (Ven's, "He said to take two grown ups" was actually "He said to take my parents" in Japanese), Aqua and Terra ruffling Ven's hair at some point, Terra's wooden keyblade, Ven's sacrifice, Aqua's sacrifice, Terra still thinking of Aqua and Ven a decade later in his empty suit, Ven literally running away just for an opportunity to talk to Terra, Aqua's constant inner monologues about the two, etc etc. They've done plenty of things that isn't someone saying "We're friends!" to show or imply their friendship when day-to-day living or fighting together everyday wasn't an option.
Unfortunately for this trio, the point of the game was to break their seemingly fragile relationships and turn them against each other. Again, I think that plot point could have been done much better so that the characters didn't seem fickle, but the point got across. Despite being close friends, practically siblings, there were still some underlying issues that Xehanort was able to manipulate and keep the three apart.
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And I don't think they expect you to be sad about everyone dying because only one person died. Master Eraqus. My feelings on Master Eraqus are mixed, I feel he got too little screentime and was not referenced enough throughout the game. Obviously Terra thought very highly of him and unknowingly helped kill him. Do I feel for Terra in that case? Yeah. I definitely feel for him that he walked right into Xehanort's trap and got his body and heart stolen and has yet to be seen again.
Do I feel for Ven, who we saw get ripped into two, nearly left for dead, adopted and taken care of by Terra and Aqua only to have to sacrifice himself to protect them? Yeah. He is a sad cinnamon roll and seeing that he and Sora are so intrinsically attached makes me even more feelsy.
Do I feel for Aqua, who had to fight her two friends? Who had to take the ruins of the home she grew up in and make a tomb for Ven, and who fell to darkness trying to prevent Terra from being "lost forever"? Most definitely. I feel bad that she made so many mistakes while dealing with Terra (like Eraqus), but in the end she still decided to save his life without a second thought.
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As for the prequel syndrome, that's the word I meant to use, not prologue. Wonder where that came from... Anyway, I don't mind that they try to explore the lore. I am a lore junky and as long as it isn't too out there or stupid, I will accept it. Personally, I don't really like the design of the [chi]blade and I wish it were something that looked more natural, but I digress. Whatever is happening with [Chi] right now is probably meant to add in stuff they can use for new sagas after Xehanort's saga is over. I'd rather they do that instead of having another [Chi] like game to explain yet another saga.
I don't care about the keyblade being 'special' or any of that crap. Yes, KH1 may have implied that, but as of KH2, it's obvious that that is not the case. BbS was not the first game that introduced the idea of more wielders or even that there was a war. It was KH2. That said, I don't want EVERYONE to have a keyblade, but out of the hundreds of characters so far, only a small portion of them can wield one. That is 'special' enough.
The lore of KH is still quite vague, even if we are given specific details such as how to open KH and how to summon the [chi]blade, we'll probably never be told where the [chi]blade comes from other than that it's just naturally occuring or a force of nature. Heck, we're this far in the series and they still haven't decided to explain what exactly Kingdom Hearts is other than a mass of hearts with power.
It might seem like BbS and [chi] are trying to give us all the answers, but both games only produce more questions than answers, and I like it that way.