I had a cat who had feline leukemia, and we tried to keep him alive, with numerous trips to the vet. But, in the end, the disease just overwhelmed him. He had such a hard time. If I had another cat with the same diagnosis, then I would just have it put away immediately. Not being dismissive, but just realistic.
You are a very kind to have picked up the kitten and looked after it. At least the little creature is being fed and looked after. It is not out there frantically searching for a home. If it does have to be euthenised, this is not a cruel act - they pass away immediately without suffering.
The vet thinks the kitten has a fighting chance at a good life - if he had recommended it be put down immediately, I would have accepted it. As it is, a lot can happen in three days.
My Shadowfax tested positive for FIP, which is fatal just like FeL. I had a prayer thread on it here on FR. It turned out to be a false positive. I agree that clearly fatally ill cats should be put down, but you shouldn't put your cat down immediately because a test says so.