I've wondered about this too. It seems to me that much of the aesthetic of black metal and the concepts behind it are born in the rebellious youthful mindset. If notions of these aspects are at the root of creating the music, then perhaps an aesthetic and conceptual maturation is required if we are to expect consistent contributions instead of musicians constantly burning out on the genre and producing tainted works.
One of the defining features of black metal is its vicious nihilistic edge. This, as anus espouses, cannot sustain itself (unless it becomes fatalism); it is transitional in nature. Perhaps the maturation is rather open; whatever will follow the nihilistic clearing away that has influenced many young Hessians will be what a more enduring form of music can be based on, one that is beyond the need for nihilistic states of mind. However, nihilistic themes are so central that it's not clear if we could even call something that went beyond them black metal or an extension of black metal.
However, the frenzied states of mind and activity that musicians and listeners alike enter into do not seem so easily seperable as merely temporary features, and such states require alot of energy. Perhaps they do require youth, but maybe not. Perhaps they just require a new inspiration. Black metal is simply a dying star, as far as inspiration is currently concerned. Something longer burning would allow for more extended contributions through time, and maybe this can arise as the genre and its adherents mature if they don't succumb to the chaos they've wrapped around themselves.