Disney characters MADE this game, who could have paired better with Sora than Donald and Goofy? alot of people will say Kairi and Riku but that would mean a whole other story and IMO this game has an excellent story that would be hard to replace without Disney characters
the main reason i bought kingdom hearts at first was because i knew characters like cloud and squall were going to be in it. thats the coolest part of the game IMO, seeing your favorite final fantasy characters interact (like when cloud and leon fought the heartless together at hollow bastion). it just so happens that i found kingdom hearts to be a great game.
I know what you're saying, I would've really loved to see the games if they were pure KH storyline and characters, the thing that always got me when playing was the games were a little too "kiddey" from time to time thats all.
the disney characters mixed well with the FF characters and there was of course the actually KH hearts characters themselves who made the story "go". so yeah i quite like it that the disney and FF characters were put in a game.
this in turn brings up another question...what other two things could you mix together such as metal gear's Solid Snake meeting and working with Splinter cell's Sam Fisher.
While I did enjoy the FF characters alot in the Kingdom Hearts games, they weren't really needed or treated as important after the first two games. So I'd say Kingdom Hearts can obviously survive without FF characters.
Disney characters are much more important to the overall story of Kingdom Hearts. Like many people have said already, it would be a totally different game without them there. It would no longer be Kingdom Hearts. It wouldn't have the special feel it has now.