Exactly. Why do you think we remember them as legends?
If Paul McCartney were shot to death and not Lennon, we'd all remember Paul as the heart and soul of the Beatles. If Jimi Hendrix would have gone into rehab and lived a full life, he'd just be a very talented guitarist. If Kurt Cobain were still alive and Nirvana touring actively, I don't think they'd remain the voice of generation X.
Besides; who the hell appreciated Da Vinci's art when he was alive?
Let me add one to your list: John Belushi. He was funny, but in death he became more popular and deified than he ever was in life.
Others may correct me as I was 17 when Lennon was killed. I'm more of a Beetle fan now than I was then, but my memory tells me that while they were all alive, Lennon, more than the other Beatles, would have been regarded as the soul. Even then I think McCartney was viewed more of the businessman, real world Beatle. But I agree with you that had Paul been killed in '81 instead, his legend would have put him above all other Beatles. (Sorry, George and Ringo, you were great, but an early death wouldn't have placed you above the other guys).
Here's another one: Jim Valvano. He won a great national championship, and delivered a powerful ESPY speech before his untimely death from cancer, but he ran a sleazy program at NC State where academic failures and bending elligibility rules led to his dismissal in disgrace in the early 90s.
