I agree, bands need to evolve, otherwise they're Motley Crue.
I'v been getting into Isis. They still play together thought, most of them, as Palms with Chino Moreno, they're even gonna do another album. They probably felt like they couldn't bring the music they make with Isis any further, so they decided to create a new "band", without tarnishing the Isis name. Just my theory, of course.
But it's a theory that makes you think. How can a band like Mastodon do something new and exciting but still sound like the same band? I'm sure they can do it. Metallica did it with the Black album, and of course continued to do it with they're next albums. Obviously some think it was too drastic ;p I love their older thrash, but imagine they didn't change like they did. They would be Megadeth. Death Magnetic was IMHO great because it felt new and not stale because they returned to a style they did earlier but stopped, coming back to it from a new and mature angel. (i know, comments like these create shitstorms, if you have any comments meet me in the Metallica thread
On a slight related note though, i remember Metallica say Korn was the future of metal once upon a time. They were wrong of course. But from what I keep hearing in different place and from metal fans and musicians talking about influences, part of me feels that Mastodon may be the future of metal. I'm not exactly sure what I mean....but i think they're a very important band right now and influencing a lot of new and their fellow contemporary bands.