Arguably the most tragic loss was Buddy Holly, although Valens was only a teen when he died. The tragedies that befell Rock and Roll after this day nearly destroyed rock as a popular genre. I mean we had That'll Be the Day replaced with Does Your Chewing Gum Lose its Flavor
If these three live, it has a huge impact, Holly was clean, no marrying his young cousin, or criminal chargers were likely to be brought up on him, nor was he likely to join the Army, and during this timeframe he is going to be the head of Rock and Roll, he would certainly set the standard.
Richie Valens is going to remain popular for a couple more years, he was certainly talented, but I see him as pioneering a Latino Rock genre rather than Pop Rock. His star is likely to start fading in the wake of Beatlemania and the British Invasion.
The Big Bopper, I heard a radio disc jockey compare the Big Bopper to Ray Stephens and believes if the BB had lived he would advance along those lines. I have a feeling the BB was going to be a one hit wonder before disappearing from mainstream entirely.
The biggest effect I can see is going to be on the British Invasion, I doubt these three living would effect it too much, but groups such as the Hollies and Beatles were so influenced by Buddy Holly any thing he records after his Winter Tour could alter their musical style, and have small ripples throughout history.
The question is, does Holly get introduced to drugs in the mid to late 1960's? If so does how do they effect his music? Its hard to imagine a psychodelic Buddy Holly, or have him tripping on acid playing anti-war themes. If he doesn't get involved in drugs does how does that effect his popularity? I'm sure he'd have a following and would be like a Monkees act, only solo.