Ian McKellen Reveals Why He Returned as Gandalf
What clinched it and made up my mind was the advice of a wise friend: "Ian, all those fans of the LOTR aren't going to understand or care about your doubts. They just want to see you back as Gandalf." And then I realised what I'd known all along, that I couldn't bear to think of another actor donning the pointy hat and grey robes.
I can't imagine anyone else taking the mantle of Gandalf from McKellen... he certainly embodies the character in a way I never imagined an actor might actually be capable of after being such a long-time fan of LOTR (certainly no other actor in the films was ever as indispensable as he was--someone else could've played Frodo, or Aragorn, or Gimli... but I don't think anyone else could've played Gandalf).