It’s not a certainty. It makes sense to the “monkey chatter” in your head, but it doesn’t make sense from an evolutionary psychology point of view. The individual has no real value to the genetic processes that drive us. Individuals that hang around too long are detrimental to those genetic processes. There are biological reasons that suicide and “going off to die” is so common not only among humans, but many other animals. Outside of our ego, which contributes so strongly to our monkey chatter but less strongly to our actual behaviours, we don’t really want to live forever.

Even from a technical point of view, it flies in the face of every assumption life is built upon. Death is essential to every biological process that spans organisms. We would have to create an inhuman thing outside of biology that is neither alive nor dead. It is debatable whether that is an extension of life rather than a transformation into something else, still destroying the old thing.