I think they (correctly) feel that there is just so much demand that the middle class would quickly buy up available properties and they would be highly unlikely to hit prices affordable to the poor by market forces. This is simply because of the huge desirableness of living in CA and this weather. This would esp be true if the new units were in high enough quantity that the overall area would become safer.
And if the poor live in a rent controlled unit, they don't have any strong desire to live in a similar unit with more occupants on the same lot. They think more like an owner in that sense.
Supply & demand is a great concept but the supply will always be limited by a fixed quantity of land - whether its land in CA, land near transit, land with etc, etc. And some of them may feel the land they are on is hugely undervalued and unlikely to ever remain affordable.
I don't think your "correctly" parenthetical is apt — you are proposing that demand is infinite and therefore supply and demand does not apply. Clearly demand is not infinite because prices are finite. Also, if demand can eat up all of the supply, you are still supply-limited. That is supply and demand in action — you're imagining a scenario in which supply is still very constrained and claiming that prices will stay high. I agree! Supply needs to be less constrained than that to lower prices, which is kind of the general thrust of the YIMBYs.
Yes, rent control distorts supply and demand, leading to higher prices.
Land will eventually constrain, but at the moment SFBA has tons and tons of unused air that is kept off the market by zoning.
Your argument is not very clear to me. They may very correctly realize that when you take how large the demand is + how much land is up for sale + how much could possibly be built on it + which neighborhood they want to remain in... the chance the market will reduce those units to prices that affordable to them... the chances are very low. So its not really a surprise they become NIMBYs.
And if the poor live in a rent controlled unit, they don't have any strong desire to live in a similar unit with more occupants on the same lot. They think more like an owner in that sense.
Supply & demand is a great concept but the supply will always be limited by a fixed quantity of land - whether its land in CA, land near transit, land with etc, etc. And some of them may feel the land they are on is hugely undervalued and unlikely to ever remain affordable.