So it was estimated back when, and don't ask me where this came from, that to settle the American west would take centuries, and perhaps around the figure I mentioned in the title. I believe the big reason for this was the lack of understanding just how much industrialization could speed things up (and maybe underestimating California as a lynch pin of western movement).
The challenge is to make it so that settling the American west really does take centuries upon centuries. And also, what if that did happen?