First off, if you can't be very nearly 100% sure of failure, you should do cryonics anyway -- as long as your expected value of survival is greater than cost times probability. If you are still only 99% sure cryonics would fail, you should still be willing to bet up to $50,000 on it if your life is worth $5 million.
Second off, your argument seems to include damage and suspension under the same umbrella. Suspension as a problem for personhood doesn't make much sense, unless you are willing to admit to there being a real a risk that people who undergo extreme hypothermia also actually reanimate as a different person.
Third, repair scenarios as a risk to personhood make sense only if you apply the same criteria to stroke, dementia, and trauma victims who would benefit from similar extreme advances in brain repair technology.
Within the immortalist community, cryonics is the most pessimistic possible position. Consider the following superoptimistic alternative scenarios:
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.
This is something of a response to Charles Platt's recent article on Cryoptimism: Part 1 Part 2