James Dean was charismatic but IMO no actor. He merely "played" himself. So, Yes, he would have faded into nothingness very soon after getting too old for the "young misunderstood rebel".
that's a very good point part from in Giant he never played himself i can understand what you mean in East of Eden and Rebel without cause because of his trouble with his dad in real life.
so you think he is a very overrated actor then ? i like his acting and thought he was good but as you said in 2 of the films he was playing himself lol