19 years old and only wants yogurt

I have a 19 year old male cat who has declined recently. Things came to a head last thursday when our dog came to find us making a fuss, and taking us to the family room where the cat had collapsed. His breathing was fast and he was very weak, he had also been sick. I cleaned him up and put him on his bed next to the radiator and tried to give him some water which he reffused. Our dog spent the night sleeping next to him and washing him! The following day he seemed to have given up, I phoned the vet and explained he wasn't well enough to make the journey there and they offered to come home to put him to sleep, I couldn't do it. So things went on and I got him to have water from a spoon and tried all sorts of food that he just turned his head away from. Then my grand daughter was having a yoghurt and he liften his head and started sniffing, so I offered him some on a spoon, he loved it. He has been having a couple of spoons every two hours, yesterday he got up and started walking around again, although very weak, and this morning he got up to greet me waiting for his yoghurt. We still keep trying other food but he isn't interested. He seems brighter although weak and purrs when he sees us, the dog isn't washing him anymore and has gone back to sleeping in her bed. I doubt he will make a full recovery, but he is happy being around us, and I just wish he would go in his sleep. How long can he live on yoghurt?

I can imagine it's hard for you, but really he should have a vet look him over incase he's in pain,  because cats are good at hiding it?.

The vet shouldn't have just said he would put him to sleep either without examining him first, but please have someone look at him and let us know what they say.