Sgt Pepper was indeed a milestone....a studio album that took you on a journey rather than just a collection of songs...IMHO, it kind of helped to create the rock opera era that followed where artists embellished and further explored the concept.
Are the Beatles the reason why modern music exists
I believe that the Beatles are the reason why modern music exists. The album that ushered in modern music was "Sgt. Pepper's Lonely Hearts Club Band". Although I consider it maybe their 4th best album, this is the one(One person said it was the Rolling Stones, but do you remember what their equivalent album was? It was called "Satanic Majik Mysteries", or some such{you had to be there}.) It definitely wasn't Elvis. Although good, Elvis was not the innovation that allowed modern music. One interesting thing is to ask youngsters what the Beatles' "White Album" is.