Lars is probably the worst well known drummer in world history (he's the Nicholas Cage of drummers). He's just straight up bad, like in a bad high school band sense. That doesn't take credit away from him for creating Metallica. He's a hell of a businessman, and he knew how to get others to do something that people like, and that's a talent in its own right, but his time is just nonexistent.
You go on about Mustaine. He's a great example of a different phenomenon. He's technically quite polished as a guitarist (although playing fast is far from all there is to being good) but he's not much of a musician. His playing is boring, unimaginative, unemotional, and that, coupled with his repellent personality, is why his bands have never been all that good. I'd rather listen to Metallica play crappily than Dave Mustaine play anything. But that doesn't mean Lars Ullrich and James Hetfield are great musicians with great live chops. They aren't. Popular musicians can also be fantastic musicians (they don't have to be, though). A great example, who was a much better guitarist than Hetfield (or Mustaine for that matter), is Prince.
In any case, Megadeth, Slayer, Metallica, these guys are like Julio Iglesias. They keep singing, they have their audience, the casinos will welcome them soon.