Singers are human. The vocal cords are muscles like every other muscle in the body. Some years, everything works perfectly, other years not. In shape or out of shape.
There are sopranos in the later career years that have switched to Mezzo repertoire. They still wanted to sing, they just lost the high notes.
Are we expecting too much? It seems that we should be thankful for what we get. As I have said before, it is the only career where we make it look easy, but we are doing a hell of a lot of work. Singers are athletes and should be evaluated like an athlete.