The way HSR is planned now, it will provide some basic transportation (good), connections with potential regional service (good), and need 220mph operation over nearly the whole line (not good - requires very expensive construction) to achieve the LA-SF (OK, BUR-SJ to reach the borders of the urban areas) express time of 2 hours. Tunneling out of the Bay Area and via Tejon Pass into LA, and running down I-5 ignoring everybody else, would allow that travel time with less-fancy speeds (perhaps around 180mph/300kph); but it would provide no useful service to the SJ Valley even with a Coalinga station (!) perhaps connected to Fresno by a cross-valley rail line of some kind (no money for that). The I-5 route *was* promoted at the time of the Proposition, though mostly by non-engineers and non-planners (i.e. politicians who didn't really know what they were talking about).

The I-5 concept really competes only for flyover traffic between the Bay Area and LA, at a higher cost than the airlines have (and will have, at least until oil vanishes; and bio-jet fuel does exist) for similar service. Also, there are multiple airports in the Bay Area (including Sacramento) and LA that have similar travel times (about 2 hours from origin airport arrival to destination airport departure if not checking baggage), making a 3-hour total trip feasible for runs to many places throughout each area, not just central downtown-central downtown. IMO, that makes flyover-replacement a stupid market to limit HSR at the start - way too expensive to build for what you get. For flyover improvement, it would be better to improve transit (with something decently high speed, such as heavy rail) between the airports and downtowns so less driving is needed.

Hyperloop has the same problem. It will need to be very cheap to build and operate to be competitive in the flyover market despite the potentially incredible speed.

The current CAHSR plan (which took way too long and cost way too much, and will cost way too much due to its conflicting requirements) is more of a NECorridor concept - not just end-to-end, but local (other services using parts of or connecting with the HSR line) regional traffic too. It has the potential for making the SJ Valley a real, integrated part of California, which it isn't right now due to travel times. Perhaps part of the opposition is because some people don't want that to happen?