You make a lot of good points, but I think the life span of a chicken is closer to that of a dog or rabbit, than it is a gerbil, which is only two to four years. Goldfish also have very long potential life spans, longer than dogs. They only die at such young ages because people don't take care of them properly.
Sometimes even good keepers lose chickens, but a lot of chickens do die because of things their keepers did or didn't do. That's just the way it is, too. Plenty of dogs have died at a young age because of their owners, not just chickens. People don't get parvo vaccinations for puppies, they let them run in the street and they get hit by a car or they let them go kill a neighbor's livestock and they get shot. To me, it's the same sort of thing. Sometimes dogs die from things that couldn't be helped, sometimes it was the owner's fault. People take care of the creatures they're responsible for in different ways. It's the same with chickens.