Can't agree with you more. I also felt like the Disney worlds were a lot more relevant in the first game when compared to all the other games in the series. I mean I'm all for Square Enix original characters but I feel like the more they introduce them the more some characters (like Kairi) don't get the attention that they deserve & others like (Sora) just become exaggerated personas of what they used to be (like the more SEO characters there are the more the writers want to show how incompetent or dumb Sora is in comparison). But that is just me.
However, maybe they just might return to that way of storytelling after KH3 when the Xehanort sage ends? Possibly.