I think you're unfairly equating "signing up for cryonics" with "urging others to sign up for cryonics". If I go see a movie, I do so because I personally want to enjoy it, not out of any concern for whether it promotes good in the wider world (maybe it does, but this isn't my concern). I can later go on to recommend that movie to friends or to the internet in general, but that's a separate act.

Maybe your particular reasons for signing up are at least partially for the greater good (perhaps so you can wake up and continue the work on FAI if it remains undone), but it seems likely that most people sign up because it's something they want for themselves.