Bands That Have a Style vs. Bands That Are Stagnant

So, I don't think anyone here is a stranger to bands that never change or progress. As the old joke goes, ACDC has released the same album 20 times, but nobody cares because it's a really good album. I don't think it's a stretch to say the same joke could be made about Slayer, or Bolt Thrower, or Motorhead, or Cannibal Corpse, or Manowar. So what do people think about this sort of thing being, at least as it applies to metal? Is it permissible as bands having their own style that like to stick with? Does it detract from metal as an "art"? Are they playing it safe or just playing what people want to hear or what they want to make?