Genuinely do not understand why people are still having the "Tiana shouldn't be a thrill ride!" discussion. Song of the South was a movie of very low stakes. What about that film lends itself to being a thrill ride anymore than Princess and the Frog would? They're both children's films, I'd argue SotS even moreso, but you didn't see anybody complaining about that when Splash was still open. Is PatF more widely known to kids today than SotS was in the late 80s? Sure - but the target demographic for the attraction has never been exclusively children, and I'm not sure how what we're seeing now implies that has changed.
That's not to downplay the effort that went into actually designing the layout of the ride to fit the story of SotS / Splash. I hesitate to call Splash a "thrill ride" because while a lot of emphasis is placed onto the drops, and they are an integral part to the story itself, they play such a small part in the overall dark ride experience. I suppose there is an argument to be made about the thrills surrounding the overall buildup of tension as the ride progresses, but that's a whole other conversation to be had. Regarding the relationship of the ride layout to the story, I fail to see how the pre-existing layout will feel any less purpose built with the new theme - a theme that (at least in terms of scenic design) will remain largely the same. Whether or not those thrill elements will feel shoehorned because of the pre-existing system is yet to be seen.
Also, completely ignoring the controversial aspect, let's just pretend that Song of the South was on the chopping block regardless. If Splash HAD to be replaced with another IP, what would you suggest that would be better than what we're seeing with Princess and the Frog? Half of the movie takes place in the bayou! I challenge anybody to find a better IP to retrofit Splash with minimal changes that isn't Pocahontas (because people seem to LOVE using that excuse as if it somehow would have been the better outcome) - and yes, I am implying that what we're seeing now is still Splash with minimal changes. This isn't like Maelstrom being retrofitted for Frozen Ever After where we saw a complete shift in theme. Almost everything that made Splash Mountain iconic will still be there. I like to believe that Splash Mountain the "place" isn't changing, but rather, its inhabitants.