It's amazing how the politicians representing the people of the Bay Area don't possess egalitarian values. There's so much unused and undeveloped land available in the mountains yet the property is crazy expensive.
There's no need to develop any more open space. The south bay has plenty of dilapidated strip malls and empty parking lots that could be repurposed into high density housing.
All of that land that is undeveloped is owned by someone or something. Some of it will never be developed unless it is traded for similar land, i.e. Midpeninsula Open Space/National/State/County park land. A huge swath along 280 from Portola Valley to San Mateo is part of the water system for San Francisco - someone is going to have to convince them to give up Hetchy Hetch water before that goes away. Much of the open space near Palo Alto is owned by Stanford, and is why Stanford is one of the largest universities by size in the world. Unincorporated areas in the mountains have to supply their own water from wells and in the drought last year many places on the rainy side of the mountains went dry - building density where there is not enough water or sewage outside of septic tanks is not reasonable.
I think the question of whether a prosperous region can have both urban growth boundaries and affordable housing is an open question. The Bay Area has not set a good example in this manner; we listen to environmentalists when it’s time to put walls around the region but we don’t listen to them when they call for greater infill development (to their credit, the Greenbelt Alliance has a subsidiary called the SF Housing Action Coalition that promotes infill development, but in SF politics they are often written off as developer shills).