Go take a look at how the original cast is doing. Let's just say with the exception of Mr. Ford, they are all quite...err... comfortable.
Bringing them back now would be akin to casting Marlon Brando in the new Superman movie. Outside of wink wink nudge nudge it's not a good idea.
Well seeing as how Marlon Brando has been dead for years ( his appearence in Superman Returns was through the use of unused footage from Superman and Superman II. ) It would have been impossible for him to have been in the new Superman movie at all.