[Glirdan]

Hmmm, this is really reminding me of the last play that Shakespear ever wrote: The Tempest. Here's the general idea of what happened. The Duke of Milan was usurped by his evil twin and the King of Naples. He and his daughter were put aboard a rickety old ship and cast out to sea. However (this is the most important part), Gonazalo, the King's advisor, gave Prospero (the Duke) books and he had a staff. From the books, he learned magical abilities so to speak, which would represents his knowledge of the magic. The staff represented his way of controlling that magic. Sound kind of familiar?

I believe that the same thing applies with Gandalf and Saruman. The staff breaks, the magic is broken. Because Gandalf put so much power into breaking the bridge of Moria and the fact that the staff broke signifies (in my opinion) that he lost his magic. All his energy was spent on defeating the Balrog and because he didn't have his magic to aid him, he "died".