Your thought experiment is like saying, "suppose I win the lottery, does that disprove probability theory?" You've basically said that their total population over time is infinite, but that you've decided to focus on the very beginning of their history. A randomly selected individual from that possible world should have an immeasurably long history behind them.
Thanks, this is a good point. I can see that the argument is logically sound but it seems so unscientific - our future will be determined by physics, not a priori logical arguments - so why take it seriously?
If it's worth saying, but not worth its own post, even in Discussion, it goes here.