I think that should be left up to each individual. You might not be happy with my definition and I might not be happy with yours.
For me, (somewhat oversimplified - since the finer points are too complex to address here with any appropriate degree of brevity) I wouldn't want to exist in a state where I was unable to observe my surroundings with a reasonable degree of understanding, and/or to communicate my thoughts and wishes in some fashion to others, and/or in a state involving a continuous severe level of pain.
My nephew is here with me today. Chuck was in a severe motorcycle accident 18 years ago. He too was denied therapy for the first 10 years after the accident as his father deemed it too painful for him to endure. His father fought his mother and was temporarily awarded custody of him. His mom fought back, regained custody, and began therapy for him. For the first 14 years Chuck was on a feeding tube as he could not swallow. Today he eats with his good hand all by himself. He is paralyzed on one side still, can't speak ( grunts and moans ) and doesn't follow simple commands well ( we found that he has suffered a hearing loss as a result of his accident so we have to be very loud when we speak to him ). Because of his ongoing therapy the rigidity of his muscles from lack of movement has decreased. He said "mama" for the first time in 18 years last month.
I am glancing at him now sitting in his special chair ( my recliner, lol ) and he is sleeping so peacefully. I just changed him a bit ago as he is very heavy and I struggle with lifting him. After all he is a grown man of 6' tall and about 250 lbs, ( see, he eats very well. Too well IMO ). I do not see a man who lacks quality of life. I see a man whose life was forever changed by a horrible accident. I see a man who as an 18 year old boy found himself hooked up to life support machines. One who was pronounced brain dead and one whose mom made the agonizing decision to "pull the plug" after which he began breathing within seconds. I see a man who, although limited severely, laughs, cries, pinches my butt when I walk by to aggravate me, puckers up his lips for a kiss, drinks a beer ( oh the nerve of me ), and who loves to be talked to although he can't talk back. I see someone who has rubbed my belly when I was pregnant ( he absolutely loved to do this ) and who has held my children ( with my help ) when I placed them in his lap. He has "quality" of life. Before therapy he could do none of these things.
Once we told him that his Dr. said that he would always be a vegetable. I will never forget his response. When asked what he thought about that he simply stuck up his middle finger and laughed.
In the morning I will get him up, bathe him, put a fresh Depends on him and get him dressed for church services. Afterwards, we will go out to eat at one of his favorite restaurants and I even thought of taking him to see the movie Radio. I don't know if my back will hold out that long though. But, so much for him being brain dead, huh? BTW, he is still labeled as being in a persistant vegetative state. So much for him being a cabbage.
As for his quality of life, I ask him if he is happy. He smiles. He feels no pain. He is limited, sure. It doesn't matter to us that he may never find the cure for cancer or be a successful business person who rakes in millions. It doesn't matter that he may never father his own children. It isn't what he does that will matter in a hundred years from now. It is how we cared for him. It is the love that we gave him. The legacy of his family loving and caring for him although he is disabled and was once considered hopeless is what is going to be passed down to our future generations. My children are much better people because of having ben exposed to him. Our family is closer as a result of his circumstances. All of these things can be directly attributed to him.