Although I agree with your conclusions it's worth considering that Japan's bullet trains run 2000 passenger trains both directions every 3-5 minutes (<<1min delay) on their high traffic corridor.

That is very conservatively (2 passengers/car) 1000 cars every 5 miles with on 125ft spacing equivalent to a perfectly functioning 10 lane freeway.

More realistically (1 passenger/car) 2000 cars every 3 miles on 250ft spacing is equivalent to a 60 lane normal freeway.

For the air travel equivalent (much lower latency, but higher cost) you would need 1000 full 737-900 every hour (8000/day) compared to the total number of flights on California on a busy day, 3000.

If you build it, will they come?

While that density is impressive, Japan is the only place where that density happens. Here in Korea, we generously run a train only every twenty minutes on the Seoul-Busan corridor. I would argue that this is much higher than what is likely to be attained in California, as both Seoul and Busan have well-developed public transit infrastructure and a comparatively low rate of car ownership, which reduces the burden on riders at both ends.

(Even ignoring the TSA, 6B in ridership fees to be made up, and general messiness of American commuters.)

So now your 1300 passengers, generously, are spread out over 20 miles of freeway, instead of three.

(2600 passengers/20 miles) * (1 car / passenger) * (250 lane-ft/car) * (1 mile/5280ft) = 6.1 lanes.

Now, that was assuming one passenger per car. Add a single bus per mile-lane (seated passengers only) and this drops to four lanes. With self-driving cars and more busses, you could safely add only a single dedicated self-driving car lane and a sigle dedicated bus lane in each direction, and still have room to increase "ridership". For the price, I think that is ridiculous.

Undoubtedly if built it will get used. However: it's a nice fantasy; but in an over-committed region of a debtor nation, the implementation requires beggaring other vital programs or putting the country another couple hundred billion in debt for a public works project that employs a few thousand people to build, a thousand or so to run, and will deliver questionable economic value in a state that is already overcommitted in so many ways.

The problem with this California idea is that its proponents evoke the Bullet Train but it will never be the Bullet Train. The Japanese built that thing in 1964, which is when we should have started modernizing our own railways but didn't because GM decreed that its buses and trucks should replace railroads, which they largely succeeded. The resources for a huge project existed then; they don't exist today.

It's also worth pointing out that California is in the political dog house post-2016 elections. They backed the wrong horse, their dominant party is in the minority nationally, and they are not going to get a lot of sympathy in the Congress or from most other states who are going to be asked to foot the bill for this turkey. If the people of California want to build this thing, go right ahead; no one's stopping you. Just don't ask the rest of the country to pay for it.

California is less densliy populated than a lot of quite pleasant, developed countries. Not just the dense Japan and the UK (which are actually quite nice too) but also France, Italy, Germany, Denmark, Switzerland and even the bucolic Austria.

There is plenty of room even without population transfers to neighboring states.

"Room" isn't the limiting factor. Water is much more limiting.

Israel has 4 times the population density of California. The water shortage is because California is growing fruits and nuts for the whole country, not because of the population living there.