One Million in Four Centuries

There is a rule of thumb I use when estimating the number of descendants a person in the distant past may have today. It is based on a claim from The Mayflower Society which I have seen over the years.

It has been estimated that there are between 30 and 35 million living descendants from the 26 Mayflower families that left descendants. This would mean that on average, someone who lived four centuries ago could have between 1.15 and 1.34 million descendants today.

My rule of thumb for estimating is that one couple can have one million living descendants four centuries later. The patterns of human genetic migrations I see in the Family Forest® indicate that this number could be correct, and if not, one more century would certainly make it so.

If a couple who lived at the time of the Mayflower Pilgrims can have one million living descendants today, how many living descendants might there be today from a couple who lived four centuries before the Mayflower Pilgrims? Or eight centuries before the Mayflower Pilgrims?

What about four centuries into the future? Could it be possible that you will be an ancestor of one million people living in the 25th century?

Does anyone know of any reason why this one-million-in-four-centuries rule of thumb may be incorrect?