I will consider here no general issues, but solely the scientific issues for cryonics as I see them. Yet for us even this proves very little. What we may need to do to revive someone frozen now or in the past would consist of examining their brain and working out just what those missing connections and missing neurons were and did. It will do so not just as understanding, but by provid ing a better estimate of just what technology may be needed to revive someone. Yes, if we have that information, and the necessary computing power to work out the connectivity of neurons which no longer exist, then we may well find that some or even all of present cryonics patients and past cryonics patients have been lost completely.
Ignorance shows nothing.
Cryonics is NOT a form of science. Cryonics is an attempt to DO something, fundamentally a form of engineering. For now, we all believe that enough information survives current suspension methods that we might someday work out how to recover the patient. That's only a judgement, not a scientific judgement. And the more we realize our ignorance, the stronger becomes the case for cryonics.