"in which case there will be nobody signing up for cryonics and no market to develop the technology to defrost and revive." ...except for all that has already died while being signed up. Their wills and possibly foundations they set up would provide large enough a market if only a sufficient amount of rich people do sign up.

Another point: if we just achieved the defrosting techniques and had a bunch of cryonically suspended individuals from say the 19th century, we would be ankle deep in the saliva from all psycologists, historians etc. that would spend all their funding to get a chance to talk to such an individual.

Disclaimer: I am not signed up for cryonics. It's just that I think that some of the dismissals offered here is not well thought through.