At risk of sounding foolish, this seems odd "There can't be too many things that reduce the expected value of the future by 10%; if there were, there would be no expected value left. So, the prior that any particular thing has such an impact should be quite low."
But, if we lived in a world where there are 10 death-stars lurking around in the heavens. And, all of these are very likely to obliterate the earth and reduce the expected value of the Earth significantly. Then can't the EV detraction of each individual death star be (say) 90...
Thanks v much - your third para was a much better explanation of what I was driving at!