Demand for traveling from city to city in the US is unproven? What is 819 million domestic airplane passengers in 2023? [0] That's an average of 2.2 million people per day. Granted flying will almost always win for cross-country trips, but I'd bet a significant chunk of those flights are within a range competitive with HSR.
[0] https://www.statista.com/statistics/197790/us-airline-domest...
GP hypothesized the situation "from downtown one city to another", which is distinct from airport-to-airport.
How is it distinct in any way that would undermine their argument? Do people go airport to airport to then not drive, where people going to downtown would want to drive? Their point is that people go to other cities without their vehicle all the time with plane travel, so high speed rail would have plenty of demand up to a certain distance.