Unlike some of the 50th celebration things, Soarin wasn't "borrowed" from DL. Unless I'm very much mistaken, the one in DL is still there. So the one in WDW stays.
My unofficial guesses about other 50th stuff:
Lucky the Dinosaur: only one of him. He'll probably go elsewhere.
Cinderellabration: it's a show, so who knows. Probably depends on the show's popularity.
Lights, Motors, Action Stunt Show: they built a whole stadium for it. Like Soarin, they invested so much money into it, there's no way it's temporary.
Cinderella Castle decorations: temporary. Amazingly, all my trips to WDW- including the one this summer- have been while the castle was decorated. I have yet to see the castle in it's "regular" form!
Magical Express: may be gone altoghether, may remain but with a price, or may remain & stay free. My guess is that it stays, but at a cost. Okay, that's more a hope than a cost: we were planning on getting a towncar/limo, but the free ME makes a towncar seem extravagant. If Disney starts charging for ME, we'll get a towncar next time.