I'm not saying you're wrong, but if Disney wanted to make a film about Disney villains, they'd just do that. They wouldn't bother with KH if that's all it was.
You're right, if they wanted a film about Disney villains they'd do that. They've done that with the Descendants movies (I think, I've never seen them but I know that villains are a big part of it.)
But using the Kingdom Hearts IP helps onboarding. For some reason Disney avoids making movies about their mascot so they wouldn't pick Mickey Mouse to lead the film, leaving who else? Choosing to use KH gives the movie a protagonist to rally around that already has the boundaries for the character set.
Kairi and Riku give Sora his drive and motive through the whole game, which translates exceptionally well to film. Then there's KH unique things like the Keyblade and Heartless, a marketable weapon that Disney would properly merchandise alongside the film, and creatures that help show the danger and destruction to up the stakes to get all the Disney characters to Traverse Town and gives the characters purpose for letting Sora join their cause. It's an old trope but him turning up with the Keyblade would give any rebellion a kickstart to get back into the action of reclaiming their homes.
There's a lot that Kingdom Hearts 1 did so right to create its heroes journey and if I were an exec at Disney looking at our portfolios, this franchise that has sold 36 million units over 20 years with established concepts, arcs, and characters would be much easier to start with than trying to figure it all out from scratch. Choosing KH over an original Disney concept avoids odd out-of-character moments like that weird Peter Pan from the Chip n Dale movie.
They could throw Ansem back into the mix, especially since there's the Destiny Islands connection and we have to get to Kingdom Hearts somehow, but I think with a general mainstream audience for a kids movie, it'd be better to keep things a bit cleaner and succinct.
