We can only speculate on what caused Callas' vocal decline, yet since there was a vocal decline, I don't think we should use her to as an example why a system of categories doesn't work. In any case, I think what I term a dramatic mezzo shouldn't have a problem singing all those roles. (I would class Martha Moedl and Astrid Varnay as dramatic mezzos, as well as Callas.) I wouldn't class any of those roles as dramatic mezzo roles, though. There's no reason why an accomplished dramatic soprano couldn't give us a full bodied, interesting Rosina, so a dramatic soprano might also be able to undertake all those roles. As Balalaikaboy has pointed out having a system doesn't mean having to observe it rigidly.She doesn't prove a thing as there is no evidence that her vocal decline was caused by singing 'outside of her category', and quite a bit of evidence that it was caused by health & weight loss related issues. I picked those roles particularly because she was singing them (or parts of them in the case of Isolde) into the mid 50's. The example also points out just how easy it is to find cases that don't fit into what I consider to be rather arbitrary categories.