These questions have been exhaustively studied. For example, https://hsr.ca.gov/wp-content/uploads/docs/about/business_pl...
That analysis may be biased, but it does a decent job sketching out how to compare modes and capacities. Also, regulatory obstruction and cost inflation apply just as much to airport and highway expansion as it does to HSR. In some ways it's much worse. SFO should build a new runway, and it'd be incredibly cost effective, but NIMBY opposition was so intense SFO repudiated the idea years ago and dare not even speak of it, now.
No, that report doesn't address my point. Did you even read it? OAK in particular has been losing flights lately and is well below capacity. We don't need to build a new runway there, the existing ones are under utilized.
> that report doesn't address my point
It gives number for how much airport (gates and runways) and highway expansion would be needed to meet future demands without the HSR. And it does so apportioning traffic across the 3 major Bay Area airports and 5 LA airports.
The OAK situation only drives home the point. It's not as convenient, at least relative to the segment of people flying. Location matters. If you want to optimally utilize existing airports, we need more infrastructure, including better mass transit systems. OAK is connected to BaRT, but it's yet another transfer with about an extra 15 minutes of travel time, not including walking through the stations. And BaRT itself isn't an effective mass transit system; it's more commuter train than subway.
I'm traveling to LA next month from SF and still haven't decided whether to fly or drive. With CAHSR point-to-point the decision would be easier--take the train and rent a car.