It's one of the four park icons. Moreso even than the Chinese Theater. If they rethemed ToT, then why not Spaceship Earth? The Tree of Life? Cinderella's Castle? Yes, Splash Mountain is popular and it's being rethemed, but the reason their retheming it is because they know it'll be even more popular as Princess and the Frog. You can't turn it into Mission: Breakout, because it'll lessen the impact of Cosmic Rewind, resulting in not only a less popular ride, but two less popular rides.
One day? Maybe. Now? It's far too profitable to keep it in the same lane.
Side note: They've canonized the Hollywood Tower Hotel as the hotel owned by Harrison Hightower in the Society of Explorers and Adventurers. Remove the theme, and you rip open a gaping plot hole in Disney's attempt at a shared universe.