You do not have to be certain cryonics will work to choose it. The argument is one of expected value.
Consider the alternatives at legal death. Cremation and burial are certain and irreversible; the probability of revival is exactly zero. Cryonics offers some probability greater than zero, even if we cannot say precisely how large.
Against that, weigh the cost: affordable premiums during your life, paid with money you will not need if the bet fails. If cryonics does not work, you have lost a manageable sum. If it does, the payoff is more life. A small, bounded downside against an unbounded upside is the textbook shape of a rational bet.
None of this requires blind optimism, only a willingness to keep the option open rather than foreclose it. See also is cryonics scientifically plausible.