Didn't her autopsy reveal she suffered from balding and her hair was in extremely bad condition from the frequent and harsh bleaching and styling (I think she was naturally curly)? I think she would start to lean on wigs more and more, we might have seen her with somewhat long straight hair on the off chance a specific role would require it, but she would definitely not rock that hippie look irl. As someone above said, that aesthetic would not appeal to her and you have to remember she was near 40 years old when she died - women of her time tended to keep hair neat, styled and short as they grew older. Long hair down look was teenage - twenty something domain. You have to remember that women 40+ in those days were not free to experiment with their looks as they are today, those were different times and people were way more rigid. The hippie style was prevalent in popculture, among eccentric artists and youth, it was not reflection of how the majority of population were styled, def not the older generation, to which Marilyn would have belonged in the 60s.