Guess it depends on your definition of "quality of life". If the dog can still eat, walk and is pain free then it would be difficult to justify pulling the trigger.
"Pulling the trigger" and "putting your dog down" can be two dramatically different things.
When I was about 4 I got a beautiful blond collie type dog, which was my constant companion in rural Vermont. By the time I was about 14 she had become disabled, could barely move her back legs despite numerous visits to a good vet. She injured a back led and despite our care had maggots in it. I knew it was time, my father took her for her last walk in the woods. :bawling:
Years later, the dog that sort of replaced her was in my care, and he was old and dying and I had to have the vet put him down.
Several dogs later (which all ended up in being put down by vets after a long life) in 2011 this Corgi came into my life as a pup (technically my 28yr old son's dog).
She was wonderful. Very smart (had us well trained within 6 months), fastidious, well mannered, friendly. Would spend hours on our recliner leg rests as we watched TV.
Two weeks ago she started having trouble walking, several trips to vets, two misdiagnosis's, two MRI's and 9 days ago she was diagnosed with encephalitis and given a 70% chance. She was immediately put in emergency. Almost 1 week ago exactly, February 28th at about 3pm eastern time, we visited her in the hospital for the last time. :bawling: :bawling:
She could barely lift her head, her tongue was hanging out and she barely knew us.
Our son, who was beside himself, decided she should be put down if need be.
We convinced him to wait another day.
February 29th at 8am , (coincidentally our 8th wedding anniversary) I answered the call from the vets with a very heavy heart.
SHE WAS OK
Today, almost a week later she is still stumbling around a bit, and can't jump very well. However her recovery seems to be going well.
I could not have shot her without great remorse. However, given no vet I might have been able to administer a shot of sedative.
/edit - just go back from carrying her down the stairs (and back up) after a successful bathroom run.