IMO,classic rock covers stuff made from the late 50's to the mid to late 60's.Buddy Holly is "classic"...so is Elvis...as are the Who,Hendrix,the early stuff by the Stones,Creedence,etc.
To me,the music "died" in the early 70's...not with a bang but with a wimper.
It's not a matter of opinion. Classic rock is a recognized genre. It's guitar centric rock and roll from the late 60's through the 80's. It's important to understand this when you stroll down the classic rock isle or tune to the classic rock station.
If you want to hear Elvis or Buddy Holly, you'll have to tune to the oldies station.