To let go or keep selfishly?

We have developed a pack of 5 dogs in our home. We have always had a rotation of at least 3- so that when one dies, the other is not alone and then we start the process over again. We have a little grumpy, blind and deaf dog that is fifteen-he wheezes when he limps everywhere, but he searches all day to sit by me. Our daughter had a litter of Australian Shepherds/Corgies and she had one that she could not find a home for. We ended up bringing her home; she hasn't left our home since. She thinks she's a big dog (we also have 3 Golden Retrievers) and plays hard all day with them. One of our Golden Retrievers is only ten years old but has had several cancer tumors removed and gone through chemo and radiation. We now have her on oral chemo pills and several other prescriptions (about $300 a month). Last week, she was fragile, throwing up and having diarrhea around the clock. Dogs look into your eyes with love and always wonder why you can't fix whatever it is they need. When do you keep dogs alive for you and not their quality of life? It's difficult to be the gatekeeper of your pets' lives. I don't want that job.