We don't know what self-improving AIs and nanotechnology and stuff we haven't even figured out yet will make possible in the future. Cryonics preserve information in format that is distinct from just "any" information, like, say, if you got cremated and all that was left was ash. Little chance of coming back from that, no matter how sophisticated the technology gets.
So what are the chances of it working? Who knows, but like you say, the alternative is literally 0% chance, and this is the most important thing in your life we're talking about - your life. It's not like you can go "oh, well, I'd rather spend my money on buying a car" -- there are no alternatives to "life" either. You need to be alive to have anything else. It's the baseline.