In fact airports can be / often are built far from urban areas where land is cheaper and space is available not just for the runways and terminals but parking, cargo etc. While you want rail terminals to be in densely connected locations which are more expensive and more difficult to network to.

But my point is that you can relatively easily fly from one airport to many others. For example, the under-construction California HSR goes from LA through Bakersfield in the Central Valley up to SF and then Sacramento. If Fresno in the Central Valley grew, you could expand its commuter airport and then it would be directly linked by air to Burbank, San Jose, San Franciso etc as well as points outside the state. While the cost of adding an HSR terminal in Fresno could be large, especially if the rail doesn't run nearby (as it happens a terminal appears to be planned in Fresno, but the point remains).

I'm still a big supporter of HSR but I recognize the trade offs.