People claiming cryonics is impossible have three real lines of reasoning.
1: Hope hurts. Death has always been inevitable. When something is both undesirable and inevitable, people convince themselves it's what they wanted all along. "Who wants to live forever?" they'll say. But ask them whether they want life-saving surgery and you'll hear their real desires. It sucks to see loved ones go to sleep and never wake up. It's hard to accept our brothers, our mothers, our children, and ourselves will one day no longer exist. And one coping mechanism is to pretend it's better to be dead.
2: Ludditism. It has always been this way, and there's no reason to change. Death was good enough for my mother, for my grandmother, and for her mother and it's good enough for me. Extended life has been promised by hucksters throughout the ages, so it's always a scam. If it doesn't exist, it can't be invented. This is similar to number 1, but the emotional investment isn't in accepting death, but in accepting a state of technology not as good as the world we imagine. It's the same reason people claim(ed) strong AI, flight, the radio, nuclear power, fusion power, genetic engineering, nanotechnology, and exponential technological improvement cannot happen. Don't get me wrong, I'm not saying these things are inevitable. I'm saying there's no decent reason to say they're impossible. It's trendy and comfortable to dismiss amazing possibilities. As people discard wishful thinking they accidentally discard hope.
3: Humans are special. It's comforting and ego building to believe there's a soul or a unique form of machinery in the human brain. Too sophisticated for religion? just say "quantum". Whether by mystical energy, supernatural entities, or areas of research not yet reached it means to have a brain is to have something that transcends the known rules of the universe.
In these ways we cling to a comforting, stable worldview by refusing to ask how things could be better. These are the shackles we don.