To me, the most relevant reason for not saving for cryonics mentioned here is that the success rate of cryonics is effectively zero at present.

This does not seem to me to be a very solid objection. Cryonics doesn't depend logically on that even being possible.

I am unconvinced that people being dissatisfied with their current lives is a significant reason for rejecting this procedure.

Agreed. It may be a reason, but I don't think it is the main reason.