Replacing Springfield with Diagon Alley would be an absolutely abysmal place to put it. First of all, I think we should skip Diagon entirely and have something unique that makes USH a destination rather than just "A Smaller Version of UOR". But, setting that aside, so much traffic goes through Springfield. Even if the Studio Tour loading area was moved somewhere else, right now Springfield is the only way to and from the Lower Lot. It gets so crowded on high attendance days and the employees struggle to keep the traffic moving. Now add in a bunch of people gawking at Diagon Alley and you have a recipe for disaster. The ride is old but Springfield, as a land, is still relatively young. I think it'd be more likely that they replace The Simpsons Ride with another simpsons-themed ride than replace the whole land. I really doubt Disney is going to pull the plug on it. Even if they did, what would they do with having the rights back? Star Wars in Disneyland is already kind of a tough sell but they'd have to find a place to put Springfield, then build it, then design a new ride for it, and so many other things when just cashing the checks from Universal would be so much easier.