I own and have read a lot of books about Maria Callas, and I am now at the point where I know too much. She was a very great singer of uneven discipline even while she was married to Giovanni Battide Meneghini. Once Meneghini was no longer part of her life all hints of self-discipline went with him.
The life of a professional opera singer requires that she keep her support muscles constantly in training. Requires. There has been a lot of speculation over the years about the effect of Callas’ weight loss on these support muscles, and a lot of blame has been put here. When I listen to her singing before and after the weight loss, I don’t hear a lot of changes.
With her affair with Onasis attention to training appears to have ended.
The film closes with the concert series done with Di Stephano. I think at that time she was not yet 50. I don’t hear significant changes in her voice, though the hardness is a bit more pronounced. She could well have continued her career at this point. What I hear is huge changes in the amount of control she is able to exert over her voice due to the collapse in her conditioning. Control of the line is the key to good bel canto, and this is achieved through consistent support. By that time it simply wasn’t happening any more. Her brain remembered, but her body did not.
She was too into her own success and basically messed everything up. The film discusses this more openly than I have heard in the past. It is painful to read and to see in this video: Maria Callas, Life and Art.