With airline travel, the infrastructure already exists: the airfields, the traffic control, the hotels and car rentals and expressways. Yes, the public paid for them, but they are built and there is no annual appropriation to keep planes in the air. Ticket prices pay for landing slots and terminal rents.
Planes are more energy efficient than ever before -- airlines are scrapping perfectly good planes so they can replace them with newer more-efficient ones.
I am not opposed to investigating the Hyperloop idea; I am opposed to building the HSR (high-speed rail).
I have traveled from LA to SF many times -- actually, San Diego to the Bay Area, not San Francisco specifically. I always drove my own vehicle, so I'd have it available while there and not need to rent one. But I also stayed for a week at a time; if I was returning the same day or in a day or two, I probably would have flown. I would have traveled by train only if it was cheaper and reasonably quick. Likewise, I would travel by Hyperloop if it was cheaper than flying.