I share your feelings, watermeadow.
Our border collie, aged 15, had gone stone deaf, spent most of the night running up and down stairs barking. Always seemed anxious, trying to find one of us by sniffing our tracks . But still enjoyed her food, and shorter walks.
Our vet diagnosed canine dementia - they seem to suffer from many of the same health problems as we do.
As we were due to move back from France to England it was a blessing in a way that she died peacefully 3 months ago. But I still miss her so much.
Thinking back, it was when she became deaf, this cut off one of her main means of contact with the outside world.
Sorry to sound negative. It's difficult to weigh up the importance of their quality of life.