In “Philosophical Models of Immortality in Science Fiction,” (in: Immortal Engines: Life Extension and Immortality in Science Fiction and Fantasy) John Martin Fischer and Ruth Curl construct a taxonomy for immortality. As can be seen in the figure on the left (click for larger image), only some models of immortality meet the criterion of real personal immortality in which an individual leads an indefinitely long single life (atomistic non-serial immortality). If we leave the issue of solipsistic and non-solipsistic immortality to the side (see David Deutsch on solipsism), the only mature method listed to achieve immortality which is available right now is cryonics. Strictly speaking, cryonics itself does not achieve immortality, but it can enable a person to reach a time when technologies that can produce immortality may be available.
For many people who aspire to become immortal it is doubtful that technologies that can achieve real immortality will be available in their lifetime. But even if all “immortalists” living today would be able to benefit from life extension escape velocity, accidents do happen, and breakthroughs in rejuvenation will not be of benefit to those who are dead. Cryonics (or any form of biostasis) is not just a backup strategy to ensure that an individual will reach a time where immortalist technologies are available, it will remain necessary as long as individuals are at risk of injury that cannot be treated by the prevailing medical technologies of the time.
So far it has been assumed that immortality is possible but there are problems with immortalism. Immortality would mean a zero probability of information-theoretic death and avoidance of the most formidable obstacle of all, the heat death of the universe.
Even individuals who hope to benefit from SENS and have made arrangements for cryonics live in a world with a non-trivial probability of information-theoretic death. Minimizing the probability of information-theoretic death should be the objective of radical life extension — an outlook which itself must be balanced against values such as the quality of life.