Obviously, I haven't worked out the details, and probably won't actually go around taking advantage of these people

Hey, they'd be happy enough to still be alive, and you could donate the proceeds to eradicating polio. But unfortunately you'd also be encouraging people to take existential threats less seriously in general, which may be a bad idea. I can't decide.

Anyway, good luck finding a believer in any kind of woo who is prepared to make a cash wager on a testable outcome. Think how quickly we would have eradicated homeopathy and astrology by now! :)