That logic only holds if there's no cost, or no alternate investment. Currently the cost of cryonics is ~$28,000. If I donated that to GiveWell instead, I'd be saving ~28 lives. The question of whether I want to be immortal or save 28 mortal lives, is not one I've seen much addressed, and not one that I've yet found a satisfying answer to.
I've given it a lot of thought, and this does appear to be my True Rejection of Cryonics; if I can find a satisfying reasoning to value my immortality over those 28 mortal lives, I'd sign up.
XKCD 871: The problem of scaling the sane use of money is a problem of not crushing people's wills, not a problem of money being a limited resource. It simply isn't true that money spent on cryonics comes out of Givewell's or SIAI's pockets, unless you're Rain, which is why I'll accept that answer from Rain but not from you.
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: