We can get to that if you can establish that there's any good reason to do it in the first place.
Excellent!
Your justifications for running such simulations have so far seem to hinge on things we could learn from them (or simply creating them for their own sake, it appears that you're jumping between the two,)
The two are intertwined - we can learn a great deal from our history and ancestors while simultaneous valuing it for other reasons than the learning.
Thinking is just a particular form of approximate simulation. Simulation is a very precise form of thinking.
Right now all we know about our history is the result of taking a small collection of books and artifacts and then thinking alot about them.
Why do we write books about Roman History and debate what really happened? Why do we make television shows or movies out of it?
Consider this just the evolution of what we already do today, for much of the same reasons, but amplified by astronomical powers of increased intelligence/computation generating thought/simulation.
If you try to calculate all the possibilities as they branch off, you'll quickly run out of computing power no matter how advanced your civilization is.
This is what we call a naive algorithm, the kind you don't publish.
If you want to do calculations of the most likely outcomes of a certain event, you don't have to create a simulation so advanced that it appears to be a real universe from the inside to do that.
Calculations of the likely outcomes of certain events are the mental equivalents of thermostat operations - they are the types of things you do and think about when you lack hyperintelligence.
Eventually you want a nice canonical history. Not a book, not a movie, but the complete data set and recreation. As it is computed it exists, eventually perhaps you merge it back into the main worldline, perhaps not, and once done and completed you achieve closure.
Put another way, there is a limit where you can know absolutely every conceivable thing there is to know about your history, and this necessitates lots of massively super-detailed thinking about it - aka simulation.
Why do we write books about Roman History and debate what really happened? Why do we make television shows or movies out of it?
Consider this just the evolution of what we already do today, for much of the same reasons, but amplified by astronomical powers of increased intelligence/computation generating thought/simulation.
This is the kind of naive forward extrapolation that gets you sci fi dystopias. Most of the things we do today don't bear extrapolating to logical extremes, certainly not this.
...Calculations of the likely outcomes of certain events are
Many folk here on LW take the simulation argument (in its more general forms) seriously. Many others take Singularitarianism1 seriously. Still others take Tegmark cosmology (and related big universe hypotheses) seriously. But then I see them proceed to self-describe as atheist (instead of omnitheist, theist, deist, having a predictive distribution over states of religious belief, et cetera), and many tend to be overtly dismissive of theism. Is this signalling cultural affiliation, an attempt to communicate a point estimate, or what?
I am especially confused that the theism/atheism debate is considered a closed question on Less Wrong. Eliezer's reformulations of the Problem of Evil in terms of Fun Theory provided a fresh look at theodicy, but I do not find those arguments conclusive. A look at Luke Muehlhauser's blog surprised me; the arguments against theism are just not nearly as convincing as I'd been brought up to believe2, nor nearly convincing enough to cause what I saw as massive overconfidence on the part of most atheists, aspiring rationalists or no.
It may be that theism is in the class of hypotheses that we have yet to develop a strong enough practice of rationality to handle, even if the hypothesis has non-negligible probability given our best understanding of the evidence. We are becoming adept at wielding Occam's razor, but it may be that we are still too foolhardy to wield Solomonoff's lightsaber Tegmark's Black Blade of Disaster without chopping off our own arm. The literature on cognitive biases gives us every reason to believe we are poorly equipped to reason about infinite cosmology, decision theory, the motives of superintelligences, or our place in the universe.
Due to these considerations, it is unclear if we should go ahead doing the equivalent of philosoraptorizing amidst these poorly asked questions so far outside the realm of science. This is not the sort of domain where one should tread if one is feeling insecure in one's sanity, and it is possible that no one should tread here. Human philosophers are probably not as good at philosophy as hypothetical Friendly AI philosophers (though we've seen in the cases of decision theory and utility functions that not everything can be left for the AI to solve). I don't want to stress your epistemology too much, since it's not like your immortal soul3 matters very much. Does it?
Added: By theism I do not mean the hypothesis that Jehovah created the universe. (Well, mostly.) I am talking about the possibility of agenty processes in general creating this universe, as opposed to impersonal math-like processes like cosmological natural selection.
Added: The answer to the question raised by the post is "Yes, theism is wrong, and we don't have good words for the thing that looks a lot like theism but has less unfortunate connotations, but we do know that calling it theism would be stupid." As to whether this universe gets most of its reality fluid from agenty creators... perhaps we will come back to that argument on a day with less distracting terminology on the table.
1 Of either the 'AI-go-FOOM' or 'someday we'll be able to do lots of brain emulations' variety.
2 I was never a theist, and only recently began to question some old assumptions about the likelihood of various Creators. This perhaps either lends credibility to my interest, or lends credibility to the idea that I'm insane.
3 Or the set of things that would have been translated to Archimedes by the Chronophone as the equivalent of an immortal soul (id est, whatever concept ends up being actually significant).