Hard rock and metal bands, from Deep Purple and Thin Lizzy to '80s thrash metal and glam metal bands (Metallica, Slayer, Guns'n'Roses) right into the present have always had an appeal to teenage kids and youngsters, that's for sure, and the vibe of danger and excess (driving too fast, lots of women, sexual imagery, Satanism and occult flirtations and so on) have been part of it. Most people who love this kind of music likely picked up on it before they turned twenty-five or even eighteen. But it's not something that wears off, that's got no attraction for older and kind of serious musicians or which has only a rearview mirror appeal. There's fairly many people in thůse bands - musicians and songwriters - who will do complex and sophisticated, even groundbreaking work. Deep Purple, Mastodon and Metallica were/are outstanding musicians and they have earned the respect and interest of classically trained musicians and listeners too; of course Jon Lord was classically trained himself and there was a kind of buried influence from Bach and baroque music on Purple's peak seventies albums, not just the organ playing but in the structure of the music. I think if there wasn't more to their music than teeny rebellion those bands would not have been able to keep going with real energy and renewal past their members turning forty. With acts like Guns'n'Roses, Van Halen and INXS it was more of a "strictly for the young hotheads" thing I guess - the spirit of young upheaval, sex and drugs and rock'n'roll, was always at the heart of the show. Iron Maiden are sort of midway, it's first off a teenager appeal band but the members have clearly grown-up preoccupations, they are thinking men alright, and I think that's part of the reason they didn't crash into a ditch long ago. And you know, hard rock/metal fans are some of the most solid and die-hard in all of rock music, as unwavering as old Arsenal fans.
Pink Floyd and The Smiths are two others (though not hard rock bands, mind you!) that come to mind as having been appreciated by people in classical music too, acknowledged for their musical qualities and the improvisational, composing and playing skills of their members. I'm sure Floyd has even been a musical influence on some classical composers and players; their innovative approach to sound, sonorities, songwriting and rhythm will land them in the history books of late 20th century music some day.