Metropolitan Opera press updates announcing cast withdrawals and substitutions arrive more than once a week. It's disappointing and frustrating for ticket buyers, and Peter Gelb says he spends a lot of time talking to singers who say they are ill.
On the other hand, what happens if they go on, knowing they're not at their best? It's a question Jessye Norman was not the only one to address. Opera fans are legendary about showing disgust. And poor reviews can dog a singer for years, hurting future career opportunities.
If performers are not on top of their game, and they cancel, an inferior performance won't take place, and fans will be eager to see them when they do show up.
What about admitting illness and asking for understanding? Well, everyone listens closely for defects, fans are still disappointed, and critics mention it near the top of their reviews.
I suppose, if a singer doesn't feel good about appearing, the audience can stay on the up side by paying attention to the substitute who has been longing for this moment, and is overjoyed by the chance to be the new star. It happens.