800M is 10% of the current population - the OP's point is that if he's right, there's a non-trivial chance that he, and you, may be dead, making the prediction moot.
In terms of EV, it never pays to bet on the apocalypse since you can't collect. Markets can't price in their own demise. (it would actually be quite interesting if they could).
(Also, given how often people make predictions online, how often do you see people saying "mea culpa, I was totally wrong about that one!" It's close to zero. Guessing wrong, even in earnest, about important things, doesn't matter anymore, and doesn't pose even a minor threat to one's reputation. This lack of social/cultural corrective has made our zeitgeist is so remarkably polluted with utterly ignorant, bad faith BS like bets about the apocalypse that it doesn't even register any more. How fascinatingly horrible!)
