If Buddy Holly had lived, he would've wiped out the remnants of the Red Army with a katana and challenged Death to a guitar duel. (
Source)
Rockabilly died pretty hard after 1960. Holly's death contributed to that, but it probably wasn't going to be relevant much longer with the Boomers coming of age and turning into hippies. Holly would've had to reinvent himself like Elvis or bow out gracefully like Bill Haley. I think the most interesting timeline is the one where Holly transitions to country music in the early 60s and collaborates with Waylon Jennings, who was supposed to die in the same plane crash. Then we could've gotten the Buddy Holly outlaw country arc in the 80s and 90s.
Related unpopular opinion, in the sense that no one cares: Elvis's cover of
Hound Dog is better than the original. Nobody would give a shit about Big Mama Thornton if Elvis hadn't covered
Hound Dog.