Hating on gruesome Newsom is fun but zoning laws are a huge problem that impede more than just housing supply - like public transit become higher value if you can zone multifamily units near town centers and business centers.
But also, increasing the supply is a good way to decrease pressure on the market. There is no reason to let the perfect be the enemy of the good....especially if massive national public works projects to increase housing for poor and working class families is off the table for at least the next 3 years and 3 months.
Supply and demand are not 100% representative of market realities due to externalities, as you say. But there is a market and that market is responsive on some level (even sublinearly) to supply and demand. This is not a solution to the problem but it is a step in the right direction. Where do you want public housing to be built, next to rail lines or in the middle of nowhere? This legislation makes it possible to build public housing near rail lines by getting rid of zoning laws that previously prohibited it. Take the small wins where you can.
You see a dark cloud in this silver lining. You dont have to. Doomerism doesnt motivate action, it dissuades it.