Main use of HSR is going to be getting to and from peripheral cities. Fresno to SF and Bakersfield to LA in under an hour would be HUGE. That's going to get used more than the full SF to LA trip.
I'm not convinced the HSR is a good idea. But I actually suspect, if it turns out 50 years from now to have been, it's because it created a corridor through relatively undeveloped country (yes, there are significant cities but small relative to the endpoints) that could develop along a fast transportation link. Convenient (ADDED: somewhat) fast transit between SF and LA is almost beside the point.