But the park wasn't designed to be exclusively IP-based attractions.
Exactly. Nor even predominantly IP.
I once read a summary of Disneyland that was incredibly useful for illustrating this. Disneyland was designed to combine a zoo, a national park, a world's fair, and an amusement park/gardens. To this I would add a living history site. I found this to be a really useful way of thinking about the lands of the original Disneyland.
The original 5 lands were Main Street USA (living history site in the style of Henry Ford's creation), Adventureland (zoo), Frontierland (national park), Fantasyland (amusement park/gardens), and Tomorrowland (world's fair). These lands each had a specific purpose within the Disneyland concept.
Overtime, the identity of each of the lands has eroded. This is true for both the Magic Kingdom and Disneyland. It's difficult to tell the difference between each land besides surface level scenery. Some of that was the natural adaption to the realities of operating a theme park (animals in Adventureland would prove infeasible). But a great deal of it has been related to a lack of care and poor understanding of Disneyland or Magic Kingdom.
The Magic Kingdom is dedicated to the life and philosophy of Walt Disney. It's a celebration of his upbringing (Main Street USA), his imagination (Fantasyland), his love of progress and technology (Tomorrowland), his fascination with history (Frontierland), and his appreciation of nature (Adventureland). The basic standard for any attraction in the Magic Kingdom should be its relation to Walt Disney. Not that new stories can't be told, but that they should be harmonious with the life and philosophy of Walt Disney.
But how many realize this today? To many guests (and Imagineers!), Disneyland and Magic Kingdom are an eclectic mix of lands all surrounding a castle. You can put anything in those lands as long as the basically look right. You can also replace one of the lands with something else. Why? Because as long as there's a castle in the middle, Mickey Avenue is just as good as Main Street USA!