San Francisco is already at an untenable population density. You can have a livable city by going light or going dense, but being in the middle is what kills you (which is where SF is right now).
The light scenario is your classic suburbia, where you drive across the street. Dense is New York City, with many amenities in walking distance and copious public transportation. The middle (San Francisco) doesn't have enough people to support good public transit (buses/trains stopping every 5 minutes or less) 24 hrs a day, but has so many people that the roads are gridlocked.
SF needs to thin out or get dense if it wants to survive. What force do you think will decimate the population by 50%?
The reality is that the real blight on nature is the single family homes which are built on forests and farmland.