If you view morality as entirely a means of civilisation co-ordinating then you're already immune to Pascal's Mugging because you don't have any reason to care in the slightest about simulated people who exist entirely outside the scope of your morality. So why bother talking about how to bound the utility of something to which you essentially assign zero utility to in the first place?

Or, to be a little more polite and turn the criticism around, if you do actually care a little bit about a large number of hypothetical extra-universal simulated beings, you need to find a different starting point for describing those feelings than facilitating civilisational co-ordination. In particular, the question of what sorts of probability trade-offs the existing population of earth would make (which seems to be the fundamental point of your argument) is informative, but far from the be-all and end-all of how to consider this topic.