Just a note that [1] is really out of date - in 2025, more than 50% of Americans have a passport. https://www.americanprogress.org/wp-content/uploads/sites/2/...
But OTOH there's a chart on this page that shows a longer term trend of nearly linear growth in US passport holder percentage from the 1990's: https://lindynewsletter.beehiiv.com/p/japan-gets-there-first...