I think that every case should be taken on it's merits.
You often see a dog out on a walk that has lost a leg and copes very well. Although that's mostly when the loss has happened early in in the dog's life. Many people have working dogs. There comes a time when those working dogs are too old to work. Do we put them to sleep because they can't work? I'd like to think we don't! I had an old picking up dog until a year ago. There came a time when he couldn't get about as much and I stopped taking him. He wasn't happy, especially as I took the younger dog. But he got used to it after a while and knew to stay in his bed those mornings when I had "those clothes" on and enjoyed his lie in. When I got back, I would hide a pheasant or two somewhere in the garden and I'd set him off to find them. He loved that.
So to those who are just a tad quick to say PTS, I'd say that you have to weigh up all the pros and cons and remember that dogs can be very versatile and accommodating, with the ability to cope with disability, even loss of sight or hearing pretty well. Of course you have to look at their situation from a detached viewpoint and with the dog's best interests at heart. The dog's quality of life is the thing, of course, but just because their quality of life is diminished, doesn't mean there's none there.