From what I saw the few times I went backstage the bulk of the 'hard set' is pretty well made, steel stage and all. and the Candelabra are still the original. The drapes definitely got replaced a while ago as the one at the top of the show is much brighter than it use to be (the first one in the auction).
as for the lights, I agree that it will eventually become harder to source replacement bulbs, and those old fashioned bulbs arent cheap. Converting to LED will also save on their electrical bill, but I have yet to see a 'modern' show with LED lights that have the warmth that the incandescent lights in Phantom have. Alot of LED lit shows these days have a 'bubble gum' look to them
http://www.flickr.com/photos/27199361@N08/ Phantom at the Royal Empire Theatre