I think that Marilyn Monroe's career would've declined by the time she reached her 40s. Her success was largely based on her sex appeal, and Hollywood can be quite cruel towards aging actresses. And I don't really see her having enough acting ability to keep up with the competition in the 1970s. The directors of this era (called New Hollywood) were creating films that were far away from the glamorous roles that one associates with MM. However, I could see her doing television work from time to time before retiring from acting altogether in the early 90s.
Having said that, I don't think that her legacy would've been seriously negatively impacted had she survived to this day. Basically, her career peak cemented her as a Hollywood icon long before her death.
With James Dean, the question is whether he would have been able to avoid being typecast as a juvenile idol for the second half of the 1950s. If Hollywood is cruel towards aging actresses, it can be even crueler towards child/teen stars wanting to grow up. Often, it's the fault of the audiences, who sense a disconnect between what they expect to see and what the actors themselves want to convey/portray.
Since James Dean seemed like a modern actor who could be willing to adapt to new cinematic trends, like more realism, and because male actors have a greater chance at a longer duration as movie stars, I could see him doing quite well in the 60s and 70s. Yet, because his legacy is so closely connected to his death at a young age, he would definitely lose the icon status even if successful.
As for Buddy Holly, the circumstances are a bit different from Monroe & Dean. Since rock music was a novelty genre at the time, there wasn't an abundance of rivaling artists on the market. With him being a pioneer with a large fanbase, all it would take would be for him to just keep doing what he was doing. I have always felt that fans of music artists are more loyal on a long-term basis than fans of film actors. Don't know if it's true or not; however, I can definitely see Buddy's career going on for decades. Well, unless drugs got to him, being a rock star and all. Maybe he would've died in 1981 instead and we'd ask ourselves "what if Buddy Holly hadn't died in 1981 of that drug overdose?"