"kingdom hearts 3 is bad because the Disney worlds aren't like they used to be in KH1 and KH2" wrong kingdom hearts 3 is good exactly because the way soras involvement in the Disney worlds changed. the way he doesn't quite fit in in the Disney worlds anymore is meaningful because that's an indicator about how much sora has changed throughout the series. he doesn't fits in like he used to when he was younger, he doesn't connect like he used to when he was younger because as you grow up friendships stop just being meeting up in the playground and playing together and saying you're gonna be best friends forever and start being about choosing to be friends over everything that happens to you and your friends and all the bad things about it and making an effort to connect with them and their feelings. he goes along in the Disney worlds but he isn't as vital as he used to be because he's grown up. arguably,.the only character he truly became friends was Rapunzel through making her happy (using aero on the flowers, splashing water on her, bringing her the birds) and even then he didn't get involved in the plot as much as he used to get in KH1 and KH2. The Pixar worlds and big hero 6 world are worlds that don't follow along the plot of the movies, but toy story touches the theme of growing up when sora talks about andy- (which would later be followed up in the next toy story movies with Andy growing up), big hero 6 touched on the topic of loss- something sora never really experienced as much before because he always got people back- Riku came back, Kairi came back. Although the world hinted at Roxas, it is pretty neat how sora would proceed to actually lose Kairi next, even if he did all that to get back after. Monstropolis has Sora helping Mike and Sully protect Boo, a child, keeping her safe and maintaining her innocence. He never made any meaningful connection in the frozen world, only watching the plot as it goes- kinda like how when we grow up we kinda lose touch in what kids like nowadays. The pirates world is one sora already knows, so he doesn't need to make any connections there. However, the scene where Davy Jones strikes Will once again brings the topic of loss- and the world also brings up the topic of love. Olympus is another world that returned, with Hercules deciding to not become a god just so he can be with Meg, the person he loves. Once again, love and loss are something that feel very different from when you're a kid and when you're older. Sora is learning that for the first time in kingdom hearts 3. The cherry on top of this whole is 100 acre woods- a world about a childhood icon, Winnie the Pooh. The world starts with Merlin showing sora how he's not in the book anymore, and sora goes inside the book to find out what has happened. Nothing seems really wrong with the world like previous games, but sora decides to help the harvest. Immediately after the first minigame sora wants to leave the world- like he's too old for that stuff now (which might also be why the minigames are the way they are now- Sora's heart just isn't in it anymore), and Pooh insists that he must stay. In the final cutscene, Pooh and sora talk about how their connection changed, and even though sora pretends everything is alright, he knows it's not. He knows that his connection with Pooh faded, and when talking with Merlin he shows how worried he is of losing that connection- because that connection is like our happy childhood innocence, our memories. When we start growing up, everyone worries about losing what we had in our childhoods. Merlin then proceeds to say that the connection won't be the same, but new connections to the same people can be formed. We can't connect to our childhood and people from our childhood like we used to connect when we were kids. But because people change, the connection can be formed through a different way, even if it isn't the same.
Donald and Goofy are what represents that connection to childhood. Even as his relationship with them changed throughout the games, their friendship continues to be strong. He walks with two childhood icons, something so meaningful, that shows that you can always maintain stuff from your childhood in a happy way- even, that you should maintain the connection and not let the boring growing up adult life take that from you, making you lose yourself in the adult life. That's why it had to be Goofy and Donald on the final battle, instead of Riku and Kairi. Sure, Riku and Kairi are his childhood best friends, but the message to the player would change a bit because they're not the players childhood best friends, while almost everyone that was a kid once has a connection to Donald and Goofy.