> If only there were some other high-value use for San Francisco real estate besides office space.
Converting office space to residential space is hard. Not just the zoning issues, these can be solved... the major problem is building technology. The entire HVAC system has to be ripped out and replaced, as people want individual temperature controls, to not smell whatever their neighbor is cooking or smoking and they want to have showers as well, which is problematic because office HVAC systems are not set up to deal with high humidity air and will have instant mold issues. (Side note, that is the reason why many companies, despite offering "bike to work" incentives, won't have showers installed)
Some spaces, particularly high-rises, lack windows that can be opened. A lot of space can't reasonably be rented out as many jurisdictions require a certain amount of direct sunlight for a room to be legally considered residential.
But the worst issue is plumbing: offices generally are set up with centralized loos and the kitchen nearby (if there is a kitchen at all). That keeps the amount of vertical pipes for cold, hot and waste water very low. In a residential building however, unless it's some sort of boarding house, people generally expect to have their own bathroom which means a ton of work to retrofit, more so in a way that doesn't lead to noise complaints.