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We only need to be concerned about actual civilizations, not all possible civilizations. We don't know how many actual civilizations there are, so there could be very few (we know of only one). We also don't know how difficult creating a sufficiently realistic simulation will be - obviously the more difficult it is to achieve such a simulation the more likely it is that civilizations will tend to go extinct before they create simulations. Finally, Bostrom's propositions use "almost all" rather than "all", e.g. "almost all civilizations at our current level of development go extinct before reaching technological maturity".
In light of these considerations, Bostrom's first two propositions do not seem implausible to me.
It all depends of our understanding of actuality. If modal realism is true, there is no difference between actual and possible. If our universe is really very large because of MWI, inflation and other universes, there should be many civilizations. But it all require some difficult philosophical questions, so it is better to use simple model with cut-of. (I think that model realism is true and all possible is actualy exist somewhere in the universe, if actuality is not depending of human consciousness, but it is long story for another post, so I will not co... (read more)