I think that some version of the sunk cost fallacy might be at least part of the "threatening" feeling people get from talk of major life extension. People invest a lot of emotional energy toward accepting the inevitability of death (and a lot of mental energy toward rationalizing it as not just inevitable but desirable), both their own eventual death and the past and future deaths of their loved ones, and being told that people might not have to die after all is like being told that you wasted all that agonizing soul-searching and grieving. (Or, worse, like being told that you wasted your loved ones' (future) lives by not convincing them to sign up for cryonics — I can easily see why a person would rather believe "All must perish, that is good and natural and right" than "My mother could have lived and I did nothing".)