I do think that there are major differences between animals and people. Each of us has to trust at least to some degree about that it is a personal decision when to draw the line. It is very difficult at times to know when enough is enough, and always different with each animal. I have met people that will out their feelings and needs ahead of what is in the best interest for the animal. That often makes me a bit crazy.
I have spent a 160.00 on a roo that I knew was old, and had lived a long and happy life. We had just come through a time where we had lost a number of animals to predators, with the worst being the pygmy goat that was killed by a neighbors dog. So as I was standing there crying, shaking, and trying to figure out how to get the safety off the .22. (I know, but that was the only way I figured out I could do it at the time.) When one of the kids said "Mama can't we just save one?" Well that had me finding an avian vet in Vancouver, and making the run across the city. Poor Adolf only lasted a short time after this. As it turned out we had managed to get a roo in the batch of chicks that we had gotten that year. Mussolini had been the one to kick Adolf's behind. We first thought it had been a hawk or something. We didn't figure out all of this until it was too late. DH was home when the next round happened. He was able to finish off Adolf, and he also did in Mussolini. (DH cried afterwards.) We were much less experienced back then. We have toughened up a lot since then. So that is my expensive chicken story.
I have since learned how get the safety off the .22, and know that it was way over kill for the deed. I do know that there are a number of people on the forum that have special needs chickens, and the birds are care for and loved as much as any chicken can be. I have also learned that sometimes it isn't worth trying to keep some of them alive. I have found that birds are rather fragile and will die for no apparent reasons, it is often better to let them go and concentrate on the healthy birds. The flock that remains will be often stronger and healthier that way.
For me I don't let my feelings for the animal become more important as their quality of life. I struggle with my Big Dog's quality of life, because I know that he is always having trouble with his skin itching. For me I know what it is like to itch all the time, it drives me a batty at times. So I worry about BD's itching, and Puppy has learned how to tell me when the itching gets unbearable. I have been getting him cortisone shots about once a month for the past 3 or 4 years. I know that cortisone isn't good for him, but the alternative was to put him down. I didn't think that he would live this long, and so I didn't think that we would be doing the cortisone shots this long. It turns out that he is still seems to be very healthy. He may be having some vision issues, and he does sleep a lot now, but given how old he is I am not surprised. I will keep him with us as long as he continues to be comfortable, even if he is cranky sometimes.
So I don't want to seem as though I don't think that sometimes we shouldn't spend some money to keep our pets healthy, I guess it is more about knowing that sometimes the answer isn't going to be the way we want it to be, and it's okay to let them go if we need too.