In a Big World, which this one appears to be on at least three counts (spatially infinite open universe, inflationary scenario in Standard Model, and Everett branches), everyone who could exist already exists with probability 1.

Infinite reasoning has many issues. Since we can only have finite amounts of evidence, then there will always be an "X" such that the probability of "the universe is of size X or bigger" becomes tiny - swamped by uncertainties in our reasoning, even the possible uncertainties in our logic (by the way, if this is wrong, and actual infinities can be demonstrated from finite evidence, please do let me know!).

It seems that the best is to simply take our decision based on the finite evidence that we can see.