Would people set a higher odds of success cutoff for starting a cryonics argument than they would for an extension of life treatment?

I would. Advocating cryonics, a known minority belief, has the chance to blowback and make you look like an ambulance-chaser or worse. Even if the dying person would calculate that cryonics is +EV for them, that doesn't mean that it's a good idea for someone else to try to intervene and get them to sign up (unless that other person is perfectly altruistic and doesn't mind the possible negatives they might suffer).

Advocating some horribly painful low-value - yet medically approved - treatment or heroic measure, on the other hand, has no downsides for you.