You give government right of first refusal on any low density property for sale in california, you get rid of sfh zoning, re-zone anything within 1Km of public transport as high density, and then build mid-rise high density buildings everywhere as you massively expand public transport network.
Within a decade you could have dense efficient cities all over california.
So you think your plan is going to work in, say, Alpine county, CA, with its 1.6 people per square mile? Its largest city has 163 people in it.
How is a high-density mid-rise apartment building going to help the people there? Do they really need a train or bus system there? How are you going to make their community better by getting rid of SFHs?
California is a big place, and a lot of it is very, VERY rural. There's no one-size-fits-all fix. San Francisco solutions aren't going to work in rural CA.