https://ss2.climatecentral.org/#12/37.7341/-122.4540?show=sa...
(The East Bay and Palo Alo are screwed, but that's not the subject here.)
Also, expensive high rises benefit the working poor by giving rich yuppies a place to live where they don't bid up the same housing that working-class people have been living in for decades. If you put up luxury condo highrises in SOMA where tech workers could walk to their offices, for example, then maybe they wouldn't be bidding up the price of 1BRs in the Mission to $4k/month. Landlords are generally unwilling to let housing go vacant, because even a 2-month vacancy can destroy much of their profit for the year. So one way or another, if you build enough housing the price will adjust itself to the point where everybody gets housing.
It has to happen eventually. As long as there is economic activity, there will be jobs hiring the best talent from around the world, spurs more economic activity, etc. That's just not going to change, demand will continue, so cities really need to get rid of zoning for height so supply could be built near the job centers. The alternative is sprawl, which means people need to be transported, consuming resources, etc. Infrastructure is also all the more difficult to build out over sprawl. In particular for California, adding to the sprawl could also mean building into wildfire prone areas.
California really does have a major problem now with wildfires and encroachment. I was amazed how fast the Camp Fire moved (an acre a second burnt at one point). There's just no way to really escape something like that. This is why I think building down as well as up is a good idea for many cities, Californian cities may have dinner different issues due to their tectonic activity but it should at least be explored.
I agree with your point regarding density in general. I always find it silly when people say America doesn't have space for more people. We have entire states with less population than some cities. I think improving infrastructure between the "flyover" states and coastal states might be one improvement that's overdue, but I don't know what the best solution would be off the top of my head.