Compared to the reducing human reproductive rate this is an important point. It's quite likely that somewhere in the regime of 12 billion is all the alive humans there ever will be - even with immortality technology. After all, how many times per 1000 years would you want to have some more children? How many of those people are likely to exist compared to a dormant mass who don't have children?
Now factor in a lack of FTL travel. If it takes thousands of years to go exploring, how many explorers are there likely to be, and how likely are they to stop and setup colonies instead?
