Or, if you don't care about offspring, consider this: the one-in-a-million chance of near-instant death is actually a bigger cost each time, since you're throwing away a longer potential lifespan. The richer you get, the more cautious you should be about throwing it all away on one roll of the dice.

Upvoted for cleverness, but I don't think that actually works. The expected loss grows at each step, but it's always proportional to the output of the last tetration step, which isn't enough to keep up with the next one; -1 * 10^10 + 10^(10^10)) is a hell of a lot smaller than -1 * 10^(10^10) + 10^(10^(10^10)), and it only gets worse from there. The growth rate is even large enough to swamp your losses if your utility is logarithmic in expected life years; that just hacks off one level of exponentiation at the initial step.

I don't see a level of caution here that allows you to turn down this particular devil's offer without leading you to some frankly insane conclusions elsewhere. In the absence of any better ideas, and assuming that utility as a function of life years isn't bounded above anywhere and that my Mephistopheles does credibly have the diabolical powers that'd allow him to run such a scam, I think I'm going to have to bite the bullet and say that it's not a scam at all. Just highly counterintuitive.