"what is the likelihood that we care about dead frozen people in the future?"

I wondered that as well when I first heard about cryonics. It is true that society in general won't care about frozen people in the future. But that isn't necessary for cryonics to work. Rather its enough that cryonics organizations care about frozen people.

Why would they care? Because the people running the organization have a vested interested in making their clients live. Among other reasons, the people running the organization might one day be clients as well, so they have to care about the success of the project.