Actually, Bostrom did not argue that we are most likely living in a simulation. Instead, he argued that at least one of three propositions must be true, and one of those propositions is that "almost all people with our sorts of experiences live in computer simulations". But, since it is possible that one of the other two propositions could be true, it does not necessarily follow that we most likely live in a simulation.
In fact, Bostrom has stated that he believes that we are probably not simulated (see the second paragraph of this paper).
Of course, per your comments, it is possible that Bostrom only said that we are probably not simulated so as not to terminate the simulation :).
In fact, first two propositions in Bostrom's article are very improbable, especially if we include in the consideration all other civilizations.
1) "All possible civilizations will never create simulations" - seems to be very implausible, we are already good in creating movies, dreams and games.
2) "All possible civilizations will go extinct before they create simulations - it also seems implausible.
If it's worth saying, but not worth its own post, then it goes here.
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