It helps to separate cryonics into two parts: preservation today, and revival in the future.
Preservation rests on well-established science. We know that cooling halts chemical decay, that vitrification avoids ice damage, and that the brain's structure carries memory and identity. Tissue has been vitrified and rewarmed with structure intact in laboratory settings.
Revival is the genuinely speculative part. No one can revive a cryopreserved human today, and doing so would require advances in cell repair, possibly molecular nanotechnology, that do not yet exist. Cryonics does not claim these will certainly arrive. It claims that preserving structure keeps the option open, whereas the conventional alternatives close it permanently.
For the reasoning behind treating it as a bet, see the case for taking the bet.