It is not a good idea to try and predict the likelihood of the emergence of future technologies by noting how these technologies failed to emerge in the past. The reason is that cryonics, singularities, and the like, are very obviously more likely to exist in the future than they were in the past (due to the invention of other new technologies), and hence the past failures cease to be relevant as the years pass. Just prior to the successful invention of most new technologies, there were many failed attempts, and hence it would seem (looking backward and applying the same reasoning) that the technology is unlikely ever to be possible.