i know cats dying is sad but trust me when i say you dont want your cat to be immortal
because immortal means that they will see the death of everything and witness the infiinite loop of the death and rebirth of the universe with a slight change each time, and eventually they will get bored of it or have extreme trauma from seeing eldritch horrors beyond any being's comprehension in an alternate reality after this one was destroyed. And if it's just plain immortal without the added no pain felt then they would feel all the pain whenever the universe was destroyed or created again