Hmmm... I don't want you to feel bad about your cat, but we had a cat that we wanted to save.... around here, feral cats have an incredibly high rate of feline leukemia.
Our local quack vet vaccinated our cat for it. When it was sick, our local quack vet couldn't figure out for 3 days (ka ching) what it was (but ruled out feline leukemia because it had been vaccinated) We had to transfer the cat to a regional veterinary center (ka ching) where they finally diagnosed feline leukemia. They found a tumor in an xray (ka ching) that was blocking the thoracic duct. The cat could have lived, but it'd have had to go through painful needles every other day to remove fluid from the thoracic cavity. We finally decided to end it's life humanely (another ka ching). In the end, it cost almost $800 for all the bills, but the outcome was the same - it was the cats time.
I love my animals. I'll gladly pay for routine visits, treatments for what ails them. But, when it's their time, it's their time and I won't pay to prolong the suffering.
Oh, and for several above, the average cat's lifespan born in the wild, at least around here (southwestern NY), is considerably shorter than other cats - feline leukemia being but one reason for it, regardless of their care later down the road.