Ian McKellen Shall Not Pass On 'Hobbit'
"Could I let Gandalf go? Would anyone else care if I did? Elsewhere, does anyone care that Michael Gambon was not the first to play Dumbledore?"
Come to think of it, he kind of has a point. But with Michael Gambon tied up with Potter, that only leaves Michael Caine. Yes, Michael Caine as Gandalf would be hilarious (get to work on that Steve Coogan) but it wouldn't serve these films very well.
In the end, McKellen has thankfully decided to return. Next time you see him on the street, yell out something along the lines of "Yo! Gandalf! Thou shall not pass!!", or something like that. It will make him feel appreciated. (McKellen's site)