Rock music is a relatively young form of entertainment. Give it another 50 years and that average lifespan will increase a great deal.
While many entertainers of the fifties have passed on, most today would be in their late seventies or into their eighties, which is average life expectancy. Many if not most rock entertainers of the sixties are still alive, and they would be approaching their seventies now.
And yes, many entertainers do die of natural causes, and I don't believe it's because of bad lifestyles alone or even at all. People do get cancer and heart disease in the general population, and many get them at an early age, unfortunately. I think of people I have gone to high school with. More of them are now dying of natural causes.
True, some pressures force some entertainers into substance abuse, but plane crashes, car accidents, and the like will increase in any occupation where substantial travel is required.