I recall that not long after The Beatles released Sgt. Pepper, Leonard Bernstein, conductor of the NY Phil., did a great program on how it followed the classical symphony form. Then he demonstrated how various segments were in fact classical music. So a lot of Classic Rock was influenced by that album and some even followed that form, even the Beach Boys and Rolling Stones followed. I guess musicians and composers are drawn to the long forms vs the 2-3 minute tunes that preceeded them.... Bolero is a favorite. been a fan of classical music for about 64 years now. That and classic rock and roll.