Rock music of today has turned to shit because of MTV's TRL, the billboards, and Brittney Spears's record sales have something to do with the size of her breasts. Rock music from the 70's through today will be remembered for years, but it lacks the true timelessness of classical music. Classical music has beautiful harmonies that be appreciated for thousands of years. Modern rock is a shallow reflection of the time, showing lack of development due to sell out bands making music for money and not to make music. Classical also has the advantage of being highly structured music. Having a distinct structure and showing development from beginning to end helps make a song timeless. Also the depth and complexity of writing for tens or even hundreds of instruments will make classical music appreciated by people for endless expanse time.
The harmonies in classical orchestral music are impossible to reproduce with a small band of rock instruments.
Modern rock has harmonies just as all music does but the harmony between the lead guitar, bass guitar and the keyboard is no where near what can be produced with an entire orchestra. Listen to Beethoven's famous "Fifth" to hear the distortion that a soli of violins produces and what that can do for the sound. Then listen to something like Green Day or Korn. There is still distortion with the string instruments but with only two or three guitars in a modern rock band the depth of the sound is lost.
Classical music is typically very structured and organized. Mozart wrote all of his music with a very distinct introduction, body, and conclusion. Listen to the introduction of his Requiem Mass and the distinct conclusion of "Eine kleine Nachtmusik." Mozart's music generally obeys the strictest rules of the time...