You seem to be missing the point. Is Song of the South relevant or known by most visitors? No. But Splash Mountain is still a great ride. Like Diagon, one of the best features of this land will be the visuals. That will be an attraction unto itself.
Haunted Mansion, Pirates of the Caribbean, Jungle Cruise, etc would have never become classics had WDI not been creative and thought up the ideas. That's what it's all about, creativity, which is what you, in the end, experience. You are experiencing WDI or UC's creative art in real life. Just because it may be based off of an IP means very little. The IP is to attract an audience and give WDI something to work with right away. Who knows how big the audience actually is, but I doubt it actually matters in the end as what really matters is the experience of the land and how they execute the attractions going into the land.
Basically, creativity is what makes a great theme park land/attraction, not the IP.