From a national perspective it hurts the overall economy. According to McKinsey the housing crisis costs California $140 billion annually in lost economic output.
From a Bay Area perspective, and as one who owns two homes here, I want the best and brightest to continue to come here to work and live. That's not going to happen if they can't afford to live here. We can look at many City's that were on top decades ago that aren't today. There's a bigger picture involved here than just older home owners trying to maximize short term values.
What's ironic is there people who paid under $100K for their homes that are now worth over $2m arguing against new development. Again, there's a bigger picture at play.