And those I have seen are mostly bands known for their music and well as their vocals and not individual vocalists. My experience is different than yours - I am usually quite impressed by how good they still are - NOT what they used to be but still quite good. And although not being quite as good - - my respect and memory fills in the gap... I enjoy it.
Singers now in their elder years I have seen mostly on TV specials, not in person, and here I think I would agree with you... and it's sad to see that nature has taken it's course. I am anything BUT put off by that, however. I guess I admire the fact that they are going down fighting and not just feeding pigeons in the park.