My concern is more about the ripple effects. Many low-skilled workers rely on service work, and servicing corporate real estate is a real industry (janitorial services, managerial duties, parking lot attendants, plumbers, even construction). They are already having a pretty hard time of it, and if this industry vanishes, "moving on" may mean going out on the streets. San Francisco, Los Angeles, New York, and San Diego have enough homeless already.
And, if it doesn't, in the absence of work they always have the option of moving somewhere with a more attractive income to cost of living ratio.
Much harder for a company to up and move than an individual or a family.
I wouldn't be surprised to see established companies expand on cheap real estate and start offering closed offices to employees.
As much as I like closed offices, I think that companies that want to save money still won't offer them. The cost of closed offices isn't just more square footage; they also incur construction costs that open office space don't: walls, doors, their own heating/cooling/ventilation ducts, their own lighting and electrical wiring, etc. And an office layout with closed offices is more expensive to modify.
Have some markets seen ~15% vacancies before? Yes, corporate real estate is very cyclical, no-one builds for years, and then the market adds 40-50% in a few years. But when it gets to 30-40% then you are probably looking for examples outside the US...and SF will be the worst-hit market, others should be less.
But...the world will move on, write down the value of property, assign losses, pay off remaining creditors, the world moves on...I don't think anyone could say a fault of the last ten years has been that shareholders/creditors were forced to swallow too many losses. The strength of a capitalist economy is that losses can be assigned quickly, and everyone moves on...in theory.