The discussion seems to focus around people who don't want their neighborhood to change but California has global problems such as "where to get the water" that are sensitive to the total number of people in the state.
It's even more of a struggle to provision transportation infrastructure for these people, whether or not it is public transit or more roads and parking for cars.
People forget that California has about the same population density as Germany despite being mostly uninhabitable, uninhabited, military reservations, farms larger than some European countries, etc. Sure San Francisco is not up there in density with the densest cities such as Singapore (5 million), Hong Kong (7 million people) and the Gaza Strip (2 million) but answers from those places might not be scalable to a state with 40 million people and in fact all of those places struggle with inequality and governance issues.
If you look at the numbers, California total water use is going down over time, even as population has risen. Efficiency has outpaced population growth.
And that's just residential water use, which is a tiny fraction of agricultural, which is far more wasteful, and often used to grow alfalfa to export internationally, in addition to dairy in the state.
So the total number of people in the state is nowhere near a cap from water.
As for transit, that is easily solved with density. In the vast majority of California, even in its larger cities, it is not legal to build densely enough to support transit. Which means that everybody drives everywhere, which doesn't scale at all. Transit and density with mixed use solve the problem in two ways, by both greatly reducing the need to drive for miles for any small errand or social event, and replace it with means of transit that are orders of magnitude more efficient than cars.
Except isn't density the reason why they can't build HSR between LA and San Francisco but only between Fresno and Bakersfield?
Attitudes have a lot to do with why transit "doesn't work" in California. People who've never been to LA think it is "sprawling" but it "sprawling" because it is large, compared to other American cities it is highly dense, it is as dense or denser than the outer boros in NY.
One bad attitude is that "nobody takes the bus" but if you ever took the bus in LA you'd find it is crazy crowded.
It is not bad to take the bus from Beverley Hills to Hollywood or take the subway to downtown and then express bus to the airport (in the HOV lane at high speed with a view that shows you what inspired The Jetsons.) There is no bus that stops on Rodeo Drive, since there is a social stigma about the bus, but the bus does stop on the next block over so the people who work there can get to work. It's that sense of stigma that's the problem.
Similarly for some perverse reason BART terminates in Milbrae despite there being a perfectly good track that is barely used that heads through Palo Alto and further south from San Francisco. Places in Germany that are similarly populated as Palo Alto have excellent train service (ever heard of the S-Bahn or Metro North?)
Granted I imagine a Utopia where the density of the neighborhood around that rail line was doubled by demolishing a single family house or two per block and putting in an 8-12 story high tower (think some neighborhoods in L.A. or some very nice neighborhoods like Moema in São Paulo) and an occasional duplex or triplex. Attitudes get in the way.
Land assembly is a huge hassle, of course. But I know HSR is going up through San Jose, and possible to SF too, and will likely get to some point in LA. I don't think they can't build there, just that it's taking longer.
For example, the San Jose to Merced section had its environment impact report approved "recently":
There is some truth in that but it is also about having a diversified economy.
Water conservation could go a long way in both the rural and urban areas. In fact, if LA could find a cubic kilometer of water storage it could be self-sufficient based on rainfall.. With the concern that you might have to clean up PoPs in manufactured products in a way that makes Prop 65 look like a mashup of Phillip Morris and the 3M corporation.
It's even more of a struggle to provision transportation infrastructure for these people, whether or not it is public transit or more roads and parking for cars.
People forget that California has about the same population density as Germany despite being mostly uninhabitable, uninhabited, military reservations, farms larger than some European countries, etc. Sure San Francisco is not up there in density with the densest cities such as Singapore (5 million), Hong Kong (7 million people) and the Gaza Strip (2 million) but answers from those places might not be scalable to a state with 40 million people and in fact all of those places struggle with inequality and governance issues.