The answer to your initial question is that Eliezer and Luke believe that if we create AI, the default result is itkills us all.or does.something else equally unpleasant. And also that creating Friendly AI will be an extraordinarily good thing, in part (and only in part) because it would be excellent protection against other risks.
That said, I think there is a limit to how confident anyone ought to be in that view, and it is worth trying to prepare for other scenarios.
Why does SI/LW focus so much on AI-FOOM disaster, with apparently much less concern for things like
Why, for example, is lukeprog's strategy sequence titled "AI Risk and Opportunity", instead of "The Singularity, Risks and Opportunities"? Doesn't it seem strange to assume that both the risks and opportunities must be AI related, before the analysis even begins? Given our current state of knowledge, I don't see how we can make such conclusions with any confidence even after a thorough analysis.
SI/LW sometimes gives the impression of being a doomsday cult, and it would help if we didn't concentrate so much on a particular doomsday scenario. (Are there any doomsday cults that say "doom is probably coming, we're not sure how but here are some likely possibilities"?)