Judging from the Japanese HSR experience, that's also the distance range at which HSR door-to-door is faster. SF to LA is juuuust outside of that distance range, sadly.
Including security theatre, a modern conventional HSR (320~350 max speed) is competitive up to ~1000km (~600 miles), possibly 1500 but that could be over-reaching. SF-LA is well within that range, it's a shorter distance than Paris - Marseilles which is a ~3:20 trip by rail (and not the most modern line in the country/world, and one with a few stops on the way).
That's assuming euro-style cities where you probably want to go from one urban center to an other though: plane requires going outside the city (to the airport) in advance due to security procedure, waiting for security and boarding, then at the destination going from airport to city center.
With HSRs, the stations are in or relatively close to city centers (usually easily accessible through public transportation), you can arrive 5mn (or less) before departure, delays may be somewhat less common (though the justification differs e.g. suicides on the track is not an issue for planes) and you arrive at the city center with better travel comfort. Especially for people sensitive to air pressure.
Fair, but you are forgetting that this is California, so even though we are beginning construction in the 21st century all of the technological gains will be put toward safety and environmentalism, not efficiency or speed; we won't get a "modern conventional HSR (320~350 max speed)". And the the HSR line is not going to be one with "few stops along the way", but with lots of stops, this is further compounded by the reality that in California city centers aren't nearly as easily accessible through public transportation as they are in Europe or Japan. LA was built around cars, and good luck getting anywhere in the valley or even the East Bay without a car.
