The problem with projecting forward is that we have no idea what trends birth/immigration rates will take over the next century. Looking historically, it's clea

The problem with projecting forward is that we have no idea what trends birth/immigration rates will take over the next century. Looking historically, it's clear that birth rates expand and contract cyclically, but this visualization seems to assume a fixed birth rate going forward so that all the hourglass trends smooth out and you end up with bottom-cropped sausage no matter where you start out.