I am in that ambivalent place right now with Bullet. His feeding tube is his life support. Without it he would not be able to eat and the choice would be clear.
Yesterday I could not get any food into him. He shook his head and the food would not go down. When I looked at his face I saw a red mass in his mouth. I came to the conclusion, achingly, with gallons of tears, that it is time to say goodby. I called the vet and later spoke to a nurse there (it was Sunday). She asked if his feeding tube was clogged, asked if he were dehydrated, said try to get some fluids into him. We made an appointment for this morning at 11:30. I said goodby to him many times, many ways.
Today I realize his feeding tube really is clogged. He has tried to eat cat food from the bowls (he hasn't been near them for weeks) and he even managed to get a little bit into himself by mouth when I opened a pack of soft cat food. I tried feeding him then, through the tube, and it would not go in. Clearly it is clogged. He has since climbed up to look out the window and come over to me, making meows. Now I am not sure I can let go.