I agree that there could be empirical evidence over this issue, but we do not currently have the evidence you mention in your first paragraph, and so we remain

I agree that there could be empirical evidence over this issue, but we do not currently have the evidence you mention in your first paragraph, and so we remain in a state of ignorance over the actual possibilities. That's a fair point about Bostrom's use of current knowledge, and his use of it in this case, to establish feasibility, is completely justified. I also have no doubts over justification in the case of arguments which say that current evidence raises or lowers the likelihood of something being so, and nor do I have doubts over justification in those cases where we say that if some proposition turned out to be true, then it would strengthen or weaken the case for a particular outcome. The only situation where I doubt the justification is where a specific numerical probability is assigned in a situation where we have no knowledge that argues for that particular value being most likely the correct one, and we are not in the process of (or about to start) an experiment or other investigation that will yield information to update that number, so that it is taken to be the final answer. The principle of indifference seems to me to be a methodological assumption, rather than some sort of law: it gets the process of quantitative investigation started, but, like Occam's razor, it can be overruled by new information. One thing that would persuade me that the numbers yielded by applying the principle of indifference are objective is if there is a case where, if some different initial value were picked instead, Bayesean updating on the basis of relevant new, correct information would be incapable of converging on the actual value. I am fairly confident that there is no such case, and that Bayesean inference is robust against initial errors, when done with adequate correct information. Perhaps one way to look at these two different responses to the simulation hypothesis is that we have started on an investigation, but are now waiting on empirical evidence, of the sort you mentioned in your first paragraph, that is not yet available. To me, it seems as if the specific numbers being used are "for illustrative purposes only" until we have some of that evidence. It may be that I am making an issue over something that everyone already understands.