It would depend on the dog and what was wrong with it. I love senior dogs. If the dog had a good temperament and was not suffering and got along with my dogs, I would probably keep it until it died or needed to be put down. Best experience of my life was adopting 2 senior Greyhounds after their owners divorced and didn't want them anymore. Giggles lived 1 yr and died from cancer at age 11. Linus lived another 6 yrs and died at 15 from cancer. I would love to one day have a rescue for senior animals to live out the rest of their lives.