Transhumanists insist that whether something is natural or not is irrelevant to whether it is good or desirable [see also our FAQs "Isn't this tampering with nature?", "Will extended life worsen overpopulation problems?", and "Why do transhumanists want to live longer?"].
The average human life span hovered between 20 and 30 years for most of our species' history. Most people today are thus living unnaturally long lives. Because of the high incidence of infectious disease, accidents, starvation, and violent death among our ancestors, very few of them lived much beyond 60 or 70. There was therefore little selection pressure to evolve the cellular repair mechanisms (and pay their metabolic costs) that would be required to keep us going beyond our meager three score years and ten. As a result of these circumstances in the distant past, we now suffer the inevitable decline of old age: damage accumulates at a faster pace than it can be repaired; tissues and organs begin to malfunction; and then we keel over and die. The quest for immortality is one of the most ancient and deep-rooted of human aspirations. It has been an important theme in human literature from the very earliest preserved written story, The Epic of Gilgamesh, and in innumerable narratives and myths ever since. It underlies the teachings of world religions about spiritual immortality and the hope of an afterlife. If death is part of the natural order, so too is the human desire to overcome death. Before transhumanism, the only hope of evading death was through reincarnation or otherworldly resurrection. Those who viewed such religious doctrines as figments of our own imagination had no alternative but to accept death as an inevitable fact of our existence. Secular worldviews, including traditional humanism, would typically include some sort of explanation of why death was not such a bad thing after all. Some existentialists even went so far as to maintain that death was necessary to give life meaning! That people should make excuses for death is understandable. Until recently there was absolutely nothing anybody could do about it, and it made some degree of sense then to create comforting philosophies according to which dying of old age is a fine thing ("deathism"). If such beliefs were once relatively harmless, and perhaps even provided some therapeutic benefit, they have now outlived their purpose. Today, we can foresee the possibility of eventually abolishing aging and we have the option of taking active measures to stay alive until then, through life extension techniques and, as a last resort, cryonics. This makes the illusions of deathist philosophies dangerous, indeed fatal, since they teach helplessness and encourage passivity.
It is too early to tell whether our days are necessarily numbered. Cosmology and fundamental physics are still incomplete and in theoretical flux; theoretical possibilities for infinite information processing (which might enable an upload to live an infinite life) seem to open and close every few years. We have to live with this uncertainty, along with the much greater uncertainty about whether any of us will manage to avoid dying prematurely, before technology has become mature.