I suspect what we're mostly looking at is a fairly predictable pyschosocial dynamic. To wit, what keeps "outsider artists" like metal musicians in the game is likely to be very different from what drew (or perhaps drove) them to their art in the first place.
Imagine you're an intelligent, creative, alienated kid (which could essentially describe most metal artists of consequence when they developed their key works) who looks out at a world full of hopeless, stupid mediocrities shuffling through life and cowering in fear of death. You're bright enough to realize that it doesn't have to be this way, that there's more to the good life than a dreary cubicle job, shopping malls and enough money for the matching mother-daughter boob job/botox package, but the morons around you marginalize you (ostensibly because you're "weird" or "antisocial," but actually because there's nothing the herd hates more than someone who reminds its members of their own insignificance and cowardice).
You carry that heroic dissonance with you, and it infuses and informs your creative endeavors, giving them a power and vitality that no one from the crowd can match (or understand). Initially, your creative momentum carries you on through a sort of manic phase of artistic productivity, and you're swept along with the cresting tide of your own ideas.
But what happens when those ideas begin to peter out, and your creative momentum slows or even begins to ebb? If you have, by some minor miracle, developed a stable, sane sense of self that extends beyond your artistic persona, you might have the wisdom to simply hang it up (Ildjarn), or at least hunker down and take some time off to discipline yourself and adapt your creative vision to a new language of sorts (Luc Lemay).
But, if you're like most, you'll probably find yourself adrift without much to anchor you. If you've been good at what you do, somewhere along the way you found that you could make a living - maybe not a king's ransom, but at least enough to get by - without having to work in a goddamn cubicle. More importantly, along the way, you gained recognition. Maybe not "fame" as the real celebrities enjoy, but a certain notoriety. A substantial group of people, all over the world, who appreciate what you do. Who validate your art, and, by extension, your existence. Maybe even a reasonably disease free girl or two who wants to suck your dick just because you're in the band. Heady stuff indeed, for an outcast. Intoxicating, actually.
And subtly, your motivations start to shift. It's no longer so much about saying something. It's not really about the art. It's about the craft. The calculation. But really, it's about staying in the game so you can get your fix. You've sold out, and you don't even know it yet.