A startup's office space is just one way that venture capital is getting funneled directly into a landlord's pocket. And office rents are rising like crazy, while pushing lots of other types of businesses out (particularly in SOMA).
If a landlord thinks they can come out ahead by evicting a long time tenant and refurbishing a mechanic's garage into office space, then you're clearly paying a much higher rent than that space used to command.
Salaries are still a larger overall expense for just about every startup, but consider how much of that employee salary goes straight to rent. 30% in pre-tax dollars is completely normal at this point, and I expect for some people it's even more. Ownership is out of the question for anyone who hasn't had a stock windfall at a previous company (or who lived here 20 years ago). When a 1 bedroom in a shitty neighborhood is over $3000/mo, there's a minimum to the salary you can accept -- even doing the normal salary-for-equity trade with a startup. Just to hit 30% of pre-tax dollars, you need to make $120,000/yr for that 1 bedroom.
There's no reason for landlords to stop until they've milked the tech economy dry. In most cases they're entirely within their rights to charge whatever the market will bear, and there is currently no shortage of people willing to pay more for less.
In the long term this will hurt San Francisco and the tech industry. The money won't last forever, and eventually the city will be so expensive that the benefits of locating here become irrelevant. SF landlords can't levy a tax on the innovation of the entire tech industry forever.