The sorts of people to whom cryonics (and large-scale longevity research in general) seem compelling are, I think, those who consider individual lives valuable in and of themselves, rather than valuable as a means to an end.

Talking about whether that's rationally defensible turns very quickly into a discussion about what the rational values to have are, which is kind of a dead end.

Also, minor note: freezing a billion people should cost significantly less than a billion times as much as freezing one person.