As a counterpoint to this, it is possible to scale existing airports up to meet consumer demand as-needed. There are thousands and thousands of airports in the US, but most don't have passenger service. Example: Paine Field in Everett (north of Seattle) became a commercial airport in 2019 to service an exploding Seattle metro. While it takes some time for regulations, it's straightforward compared to the billions needed to connect new cities with HSR.