Look, let's not dumb this down. Sure, some of Disney's attractions have delved into lighter themes, but that's not the be-all and end-all of what makes a ride successful.
I'm not saying darker themes don't have their place, but don't act like every ride needs to be some brooding, angst-filled experience to be worthwhile. Tiana's Bayou Adventure can be just as delightful and enchanting without diving into the depths of darkness. And guess what? That's okay.
And as for Magic Kingdom's supposed lack of variety, give me a break. The park's got many "dark" attractions that you listed for us all to see.
At the end of the day, it's all about good storytelling. Whether it's light or dark, what matters is how well the ride captures your imagination and takes you on a journey. So quit with the oversimplified judgments.