Maybe someone with some veterinary knowledge will be able to give more information about that aspect of things, but it's possible that your cat's kidneys were so far gone the vet realised from their own experience there was no hope. I know what you mean about feeling rushed, it happened to me once when I was told my elderly cat had mouth cancer and the vet put him to sleep there and then. I had similar worries to what you're going through, but now, about five years on, I'm just glad he didn't go through loads of suffering like some of my other cats have with prolonged treatment, and I can get it in perspective that he lived seventeen years, mostly very happy, so what difference would a few more days of feeling ill have made? Your cat would have been stressed by extra visits to and from the vets as well, so I think you probably did the right thing.