Well, I for one would argue that there's a degree of underconfidence inherent in tying the significance of metal to 'roots' in classical music.
Metal is derived from classical music. The compositional approach is the same. Rock defines the instrumentation, but that has shit-all to do with what makes the music distinctive. If I play Bach on a guitar, it is blues-based, suddenly?
Come on, grow up, open your minds. Celtic Frost admit a fundamental Kraftwerk influence, and all of death metal was influenced by progressive rock bands, who were heavily classically-influenced. Black Sabbath's guitarist played in Jethro Tull. The idea that rock music existed independently of other forms, or was the origin of metal, is the result of illiterate rockers parroting what the music industry has told them.
Ever wonder why serious historians stay out of this issue? Because the delusion layer is so thick.