Your image of James Dean is young and hot. And he will always be that way. I'm not saying I'm glad he died, just glad that he never faced the slow, embarrassing decline of many of his kind.
The worst of these is Marlon Brando. The smokin', compelling Brando of A Streetcar Named Desire or even Julius Caesar made way for the wheezing, enormous Brando of Apocalypse Now or The Godfather. Would it have been less good if he had been less fat? I doubt it.
Or Omar Sharif. Dr. Zhivago and Lawrence of Arabia make way for the saccharine and facile M. Ibrahim et les Fleurs du Coran, or the most likely atrocious piece of crap that was Hidalgo. Reduced to making bad Disney movies that star Viggo Mortensen? That has to hurt.
Or, in the same vein, Christopher Plummer. Once the strikingly handsome Captain von Trapp or the fighter pilot in The Battle of Britain, he is now appearing in cameo rôles in such gems as National Treasure, and as Diane Lane's dad in execrable John Cusack vehicles like Must Love Dogs. He didn't get fat, but talk about a fall from grace.
Can't they retire? Or, like Peter O'Toole, choose decent parts? He may have been in Troy, but he was the one bright spot in that movie. And at least he looks like he's having fun. The others are and were just miserable. It hurts to watch.