I can't remember the exact moment that heavy metal staring sounding good to me. I can only tell you that I wasn't well into my teen years and that it was in the mid 80s. Metal and punk were reactions to the status quo big rock bands of the 70s - a revolt against the mediocrity that had resulted from too many musicians, producers and companies chasing entertainment dollars in a short space of time. The result was a flood of bands who's producers had homogenized their "product" so well that it all sounded similar. The start of a movement always is fresh because something new is created from the destruction of something old. Eventually, punk and heavy metal would fall to grunge and the cycle would continue, with the music industry chasing ideas with capital - trying to maximize profit at the peak of interest.
This cycle of birth, complacency and death seems to keep repeating itself in areas outside of the arts. It follows other trends as well; in music; in TV, movies, fashion - anything where the short attention span of humanity can captured and placated. If we think of this collection of art as a reflection of our society ( which it certainly is the barometer for), then we can track at any moment the leveling that a society has before any given collapse. The collapses need not be grand - say like with the Roman empire or French Aristocracy. Sometimes the collapse is just the final breath of people having enough. It is a recognition of the end of the old - when the consumption of the fashionable ideas crawls to a halt.
I get the sense of things as I look around the Lanscape of culture that we are in this period. In movies and TV, old ideas have been repurposed and commoditized. Start Wars gets a new unit of TV shows. Marvel gets a new unit of movies. Old ideas repackaged with generic labels and sold to the masses. But the masses have stopped consuming. The desire to ingest the grotesque studio productions have leveled off. People are tired of the unoriginality. And with this collapse, I expect something new to arise.