You seem to be under the assumption that there is some minimum amount of evidence needed to give a probability. This is very common, but it is not the case. It's just as valid to say that the probability that an unknown statement X about which nothing is known is true is 0.5, as it is to say that the probability that a particular well-tested fair coin will come up heads is 0.5. Probabilities based on lots of evidence are better than probabilities based on little evidence, of course; and in particular, probabilities based on little evidence can't be too close to 0 or 1. But not having enough evidence doesn't excuse you from having to estimate the probability of something before accepting or rejecting it.

Well, right off the bat, there's a difference between "cryonics is a scam" and "cryonics is a dud investment". I think there's sufficient evidence to establish the presence of good intentions - the more difficult question is whether there's good evidence that resuscitation will become feasible.