Within the immortalist community, cryonics is the most pessimistic possible position. Consider the following superoptimistic alternative scenarios:
- Uploading will be possible before I die.
- Aging will be cured before I die.
- They will be able to reanimate a whole mouse before I die, then I'll sign up.
- I could get frozen in a freezer when I die, and they will eventually figure out how to reanimate me.
- I could pickle my brain when I die, and they will eventually figure out how to reanimate me.
- Friendly AI will cure aging and/or let me be uploaded before I die.
Cryonics -- perfusion and vitrification at LN2 temperatures under the best conditions possible -- is by far less optimistic than any of these. Of all the possible scenarios where you end up immortal, cryonics is the least optimistic. Cryonics can work even if there is no singularity or reversal tech for thousands of years into the future. It can work under the conditions of the slowest technological growth imaginable. All it assumes is that the organization (or its descendants) can survive long enough, technology doesn't go backwards (on average), and that cryopreservation of a technically sufficient nature can predate reanimation tech.
It doesn't even require the assumption that today's best possible vitrifications are good enough. See, it's entirely plausible that it's 100 years from now when they start being good enough, and 500 years later when they figure out how to reverse them. Perhaps today's population is doomed because of this. We don't know. But the fact that we don't know what exact point is good enough is sufficient to make this a worthwhile endeavor at as early of a point as possible. It doesn't require optimism -- it simply requires deliberate, rational action. The fact is that we are late for the party. In retrospect, we should have started preserving brains hundreds of years ago. Benjamin Franklin should have gone ahead and had himself immersed in alcohol.
There's a difference between having a fear and being immobilized by it. If you have a fear that cryonics won't work -- good for you! That's a perfectly rational fear. But if that fear immobilizes you and discourages you from taking action, you've lost the game. Worse than lost, you never played.