It depends on exactly what you mean by "existential risk". Development will likely - IMO - create genetic and phenotypic takeovers in due course - as the bioverse becomes engineered. That will mean no more "wild" humans.

That is something which some people seem to wail and wave their hands about - talking about the end of the human race.

The end of earth-originating civilisation seems highly unlikely to me too - which is not to say that the small chance of it is not significant enough to discuss.

Eliezer's main case for that appears to be on http://lesswrong.com/lw/y3/value_is_fragile/

I think that document is incoherent.