I'm not sure it was killed; I think it merely went underground. As I once pointed out to a music teacher, Bach is the most metal musician in history--I don't know of any other musician arrested specifically for their music! The better types of metal carry on the legacy of passion, experimentation, and beauty that classical music started. See Nightwish or Leaves Eyes for two examples. Blind Guardian's "Nightfall on Middle Earth" isn't as beautiful, but like some classical work is a tribute to a great work of literature.
I also think a huge part of the problem is that people assume love, happiness, and joy are shallow. Nothing can be further from the truth--pain, angst, and violence are the most shallow feelings a person can have, while love, joy, and happiness require true depth of character to achieve! But since everyone accepts that these are shallow, the joyful and romantic aspects of classical music have disappeared. No one wants to be caught listening to happy music, and our culture is the poorer because of it.
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