Re: the casting of LuPone and her contemporaries in the role of Carlotta
With all due respect to persistent fabulist and mortal enemy of New York waitstaff Elaine Stritch, I'm Still Here works better (solely in the context of the show) when a woman the same age as that production's Sally and Phyllis sings it. I think it's relevant that Carlotta is meant to be a contemporary of Sally and Phyllis from the same edition of the Follies, and had an affair with Ben.
Overall, I think that casting the leads as a decade or so older than they were in 1971 makes sense given how we perceive middle age differently than we did in the 70s. For example: Imelda Staunton (61), Janie Dee (55), and Tracie Bennett (56) in the 2017 London revival were all 5-10 years older than the leads from 1971 (except the younger McMartin, who was born taciturn and middle-aged). A Mulholland Dr.-era Ann Miller makes somewhat less sense.
But I'm Still Here, at least to me, speaks of a woman in middle age who is making sense of her life and career. It's not valedictory in nature - she's right in the middle of that career and having to take stock of the way she's perceived ("and then you're camp") and the opportunities afforded her.