Having a major tech industry within San Francisco, gives people jobs as well as bringing investment into the city. Taxes from these companies will help pay for roads, education etc. Having such a massive industry leave the area would mean a massive amount of people either relocating to where these jobs now are, or becoming unemployed.
No Google and the other technology companies based in the valley are not destroying San Francisco. The growth and development the tech companies actually brings new money and people into the city. The high income techies are a primary reason for the strength of the San Francisco real estate market, compared to the rest of California.
I think it is wrong to assume that the tech industry is destroying San Francisco. The tech industry has provided tons of jobs to people in that city and they should feel lucky to have such industries within their city limits. Manufacturing recessions have turned the Midwest into a rust belt, does San Fran want something comparable? I hope they wise up and appreciate what they have.
Tech industry is a pillar of development and it will be futile to redeem one for unreasonable accusation like "destroying" the city. Technology is the future of our world, it makes our life easier and much entertaining with each and every progress it makes in present as well as upcoming days.