Marx and Hegel made a big deal of dialectical materialism: something happens, then its opposite, then compromise and that makes the next level of history.
They're describing a process, not an ideal; in fact, endless compromise is a quick way to end up in the shitter.
However, it does describe THE HUMAN PROCESS, which is to zigzag between extremes because we are emotional creatures (emotions are quick responses designed to avoid intellectual lock-up for those who cannot articulate in logic a logical path).
Metal followed this.
Black Sabbath was one of those bands that no one could quantify, so the English bred it into a lower order (Led Zeppelin, Deep Purple) and came up with a rock-metal hybrid called "heavy metal" but now seen more as "hard rock."
Then there was the non-commercial response to that commercial response to a non-commercial response: the NWOBHM.
After the NWOBHM petered out, probably about the same time Def Leppard dominated the charts with "Pyromania," speed metal came around. Non-commercial response.
Then a few years later, speed metal was burnt, and death metal took over.
Then Cannibal Corpse... we all know that sad tale.
Then black metal...
Then Dimmu Borgir...
And so now we need a non-commercial response. Wonder what it'll be.