First, the only certainties in life are death and taxes. Cryonics aside, we should talk in probabilities, not certainties, and this is true of pretty much everything, including god, heliocentrism, etc.

Second, cryonics may have a small chance of succeeding - say, 1% (number pulled out of thin air) - but that's still enormously better than the alternative 0% chance of being revived after dieing in any other way. Dieing in the line of duty or after great accomplishment is similar to leaving a huge estate behind - it'll help somebody, just not you.

Third, re senile dementia, there is the possibility of committing suicide and undergoing cryonics. (Terry Pratchett spoke of a possible assisted suicide, although I see no indication he considered cryonics.)

If cryonics feels like a wash, that's a problem with our emotions. The math is pretty solid.