>In my comparison of HSR vs. cars for <300 mile travel, the rail being 'drastically less flexible' means that it's subject to the same last-mile problem as planes are, whereas cars do not have this problem. Therefore the advantages of cars for trips like this are difficult for HSR to overcome.

For personal travel, I think I'd generally agree. However, lots of US business travellers (pre-COVID anyway) would fly distances of 100,200,300 miles (the shuttles from Phila to NYC were almost always full, and that's just 90 miles!). These journeys have the no-car-last-mile problem, but that doesn't seem to have stopped the wide use of flight for those journeys.

Granted, post-COVID, it's no longer clear how many of these short-haul journeys business travellers will be making in the next 2-10 years.