Eventually, they will weaken and become prey to their predators. Many science fiction writers have seen this, and they have written stories in which small groups of humans have enjoyed biological immortality and they wind up growing culturally and morally stale. Similarly, the world's myths also feature countless stories in which heroes achieve transcendence through death. Myth and science fiction have caught onto the nature of nature. Death is necessary for life.
Death revitilizes life.
I don't, however. Moreover, I still see no evidence to change my view. In a universe where physical systems are constantly in a state of wearing down, and where organic biology exists only by indulging in enormous waste, death of individuals, an event that has absolutely no effect on the cosmos other than to provide the seeds for its localized revitilization, it is perfectly expected that biological organisms will wear out and die. I think the latter. But with this I'm not convinced that there is a gene that controls how long animals live.