Heck what if it only doubles the lifespan instead of multiplying it by insanely high numbers? If you could place everyone who is currently alive (including those suffering terminal illness) in a situation where they live exactly twice as long as a healthy person today, wouldn't you? Wouldn't that have the same or greater moral utility as saving the lives of everyone on earth from a massive meteorite strike or some such? (Assuming a few dozen breeding humans survive so it's not an extinction event.)

Cryonics could potentially accomplish this, with (according to Robin) a 5% chance. But only if it is adopted globally and soon (i.e. before such a time as they would be saved anyway, or are dead already).

One possible approach you can take for maximized altruism is simply to support global cryonics without signing up for the small-scale kind. Personally I see signing up myself as a way to lead by example (though I haven't done it yet). Cryonics is in its "early adopter" stage. The sooner it rolls out for mass production, the sooner its real benefits can be realized.