Our dogs have all been rescues. Some purebred, some not. Currently we have a 12 year old Scottish Terrier who developed diabetes two years ago, then cataracts and is now blind. We full time and she still loves to go outside and smell new places. She has all her other senses and recognizes people she knows, etc.
Yes it is hard to lose them, we get so attached. But think of how many lives we have loved and saved through rescuing and how many have lived better lives because we have taken them in when they were old and loved them for as long as we can. Nothing wrong with that.
My son's rescue was just given a devastating diagnosis for which there is no cure. He is part pit and discarded at one time. My son got him from a foster and has loved him all this time. Son got married and had a little girl who loves her dog. It will be sad for all, but at least Jax has known what love is for his senior years.
As I get older I have thought if I want another dog after my wee one dies, I might look at adopting older, unwanted dogs. I shouldn't have to worry about them outliving me and I will be able to give an older dog a great life at the end of his time.
Dale