I would say that some heavy music has stood the test of time already, to some degree. Bands like Judas Priest, Iron Maiden, Metallica and Black Sabbath, while not the hardest or fastest rockers out there, are starting to become classics. "Crazy Train", "Breakin' the Law", "The Number of the Beast", and "Enter Sandman" are somewhat known in pop culture. And if I'm not mistaken, the used the riff of Black Sabbath's "Iron Man" in the film during the credits. It's not much but at least it's something.
As for the heavier stuff, some of it has stood the test of time. Helloween has been going strong since the early 80's. "The Walls of Jericho" was a very fast, heavy album and they've gone up from their. "As Long as I Fall", one of their latest singles even spent some time on some charts. Slipknot, as little as I feel they may deserve it, is spending some time in the public eye.
It seems that being underground is an important part of metal. Refusing to be popular builds credibility and respect. Becoming popular may be viewed as "selling out", or "giving in to the man". Hell, Dream Theater almost gained popularity with their song "Pull Me Under" but they called out MTV for being the shallow, unsubstantial, whorish corporate machine that it is. Good on 'em. And as long as their are fellow headbangers who want the hardest shit to stuff in their ear-holes, or merely non-conformists who want something more, their will always be, in one form or another, something to make you deaf, in a good way.