People said that after KH1. And after KH2. And after every other game this character has so much as been mentioned in. Yes, theoretically, on paper, she "has potential." She is a PoH (which has barely any real relevance anymore), she lived on Radiant Garden (a world Nomura couldn't even include in a game it was relevant in), she was used as a pawn by Xehanort (a character who is supposed to have been done-zo), etc. etc. I've heard this all a million times over. And yet none of that has ever once translated onscreen, whether we're talking about KH1 or any other game in this series. Even if she does suddenly get a huge amount of attention now, it's not going to really be Kairi because Kairi never had a character to begin with, which is why I thought the word "facelift" was an appropriate one here. We might as well consider whoever she'll be to be a new character entirely.
I get where you're coming from also. Let's get down to brass tacks, your post comes from a place of ship wars over Riku v. Kairi, which is why I don't attach much sentiment to it other than a desire to disagree for the sake of it. Kairi would still be uninteresting even if Riku didn't exist. For me, nothing you could say changes the reality which is that there is no there there with Kairi and never has been. Nomura clearly also felt that way? I'd rather play as a character who can actually make the most of more screentime. Unfortunately, not all things the fanbase clamors for are for the better. Sort of like how all the complaining about KH1 Atlantica lead to no water / swimming environments for the next two decades of the franchise, the obsession over Kairi just means we're going to be foisted with a hollow character in a role she doesn't have the capacity to fill adequately.
If she, on paper, and theoretically speaking, had the potential to be much more than what was done with her, then she wasn't a "dud" or a "never has been". She had the character and markings to be interesting. What did Riku have that was so special in the beginning compared to her?
Absolutely nothing. He was the shounen stock best friend/rival who was a jerk and turned to evil. Literally the walking and talking archetype of it. The Sasuke to Sora's Naruto. The Blue to Sora's Red from Pokèmon (best friend/rival). Nothing unique or original. And the girl who picks and sasses the guy she likes, has some spunk, and had apparently arrived at the island rather than had been born in it like the other two we're coming to know was lesser?
Like I understand that they ended up doing nothing with her in KHI itself and games after unlike the effort they placed into Riku, that I can see the issue with, but the whole "she was such a nothing" from the get go and Riku was better? It sounds like bias to me, personally. Unless I misinterpreted your posts.
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As for Kairi's past, it's not even a Western thing. It's a general story beat that writers use to guide the audience to that particular mention so they can pay attention. How Sora brings it up, how she answers, how the camera pans behind her without getting a glimpse of her face, the moment overall...those are all narrative aspects that entails there's more to this dialogue.
Anyway, the entire spectacle is Japan-based in general. I don't know if it's because most of the directors are men, but there's this incessant trope that if a series is created, you'll have the main character and best friend/rival (both male, always, always male), whose relationship is the major focus, and if there's the unfortunate girl in the group, then she'll be treated as scraps and third fiddle. Kairi is simply another victim of this.