2019Dad,
I think you and I are sort of saying the same thing about numbers of kids. As you wrote, 2007 saw a record number of children born in the US. It was in 2008 that the numbers of babies started to drop. Looking at numbers of kids born, the years from 2010-2013 were especially low. That's why Francis' friend's "50% drop over 4 years" could be a demographics thing - if you assume kids start rec sports at age 6, and leave for club (or quit) by 11-12, the big bulge from 2006-2008 is on its way out and the much smaller numbers of 2010-2013 kids are not replacing them. But it makes sense there'd be some variation by state/city, with some places more affected than others.
Making things worse, this drop in numbers of kids is happening at the same time as there are all these other challenges to kids being outside and active. Streaming, e-sports (which my high school students try to "practice" in study hall on their Nintendo Switches), social media, etc are all huge distractions.