I was interested in these comments from an interview of Vesselina Kasarova:
WP: What is your secret in keeping your voice so fit and flexible even after two decades?
VK: The most important thing to me is the piano - crescendo and decrescendo, the dynamic juggling with the voice. If I cannot do that anymore, then I know I must have done something wrong. I want to have total control of my voice. I don’t want my voice to dictate to me what it can give. I want my voice to do what I want it to do. I am a very intuitive person. Intuition, as I understand it, is always also intelligence. Accordingly one learns from one’s own mistakes and one thinks it over. Rest time is also important for me.
WP: You also took some risks nevertheless. For example; in a single season at the Zurich Opera you sang Monteverdi, Mozart, Rossini and the Oktavian in Der Rosenkavalier . Would such a wide range of role wreck a voice?
VK: I sang my first Oktavian when I was 39 years old. Those who risk wrecking the voice on that role are ones who starts singing him at only 25. Besides I am very careful with this part. I have only sung Oktavian in Zurich and in a Japan tour of the Zurich Opera so far. And I don’t know at all whether I will sing him again. All the Richard Strauss repertoire - I never sang The Composer in 'Ariadne auf Naxos', much less the Dyer’s Wife in 'Die Frau ohne Schatten' ... that'll never happen. As far as the vocal style of speech-singing goes, I don't like it anymore.
Dr.B: Wow. I don't think I would have suggested Dyer's Wife for her in any event, but her Octavian is wonderful. It would be a shame if she never sang it again. At the end she seems to be rejecting the whole Strauss style. Try singing it like it's Mozart. That might work.
Wide range of role does not wreck a voice. Pushing, especially pushing with leaking air. Over singing. Extending into repertoire that is too high and heavy. These are the usual reasons. Wide range of role should help, I would think. She jumps immediately to discussing Strauss and ignores the other three composers as obviously not a problem. One could ask more questions here.
If one is coming to Strauss from Mozart and Rossini, perhaps he seems heavy and ruinous. I am always reminded of Gwyneth Jones who came to Strauss from Bruhnnhilde and Lady Macbeth. For her voice Strauss was a balm. I think Kasarova's voice could do a little heavy singing as long as she doesn't overdo it. My take on her is obviously quite different from her own.
Here is the link to the original interview in German.