Thanks v much - your third para was a much better explanation of what I was driving at!

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% & EV of the future remains above zero. One way of looking at it would be that the Death Stars are deployed in a staggered fashion. The impact of the deployment of the first death star being that earth's EV falls from 100 to 10, the deployment of the second would imply if falls from 10 to 1, and the third from 1 to 0.1 etc.

Apologies for the terrible explanation of my point.