A pet is a business? Look at the bottom line? I would dump that vet and question his take on his oath in a heart beat. Would you do the same for you kids, spouse, mother or father. Then why is a pet any different. A Puppy Mill is a business. They don't give one hoot about the animal. They are products of the industry. Nothing more.
Those that open their doors to an animal, should do so with all the intentions of being it's caregiver for their whole life. Care is never cheap for humans or pets.
It's understandable to question ones thoughts of when enough is enough, but only your heart can tell you that, not a vet who looks at it as a business.Your pet will give you the signs, your vet will confirm. But you have the final say. If you doubt your vet, then he isn't the right one for you. Get a second opinion. They provide the facts only. What you do with it is still your decision.
Your heart will be full of pain. Your thoughts will bounce around and you will doubt yourself over and over, but sending them to the Bridge of peace and pain free is the final and right decision you will make. Letting them go and remembering the memories of better times will be your healing force.