Ask HN: How can anyone afford to start a business in SF?

We all know the myth. The myth is a young engineer, who moves to San Francisco and starts a company. That company quickly gains traction and then applies to YC and becomes fantastically successful. How can anyone afford to do this, especially the young engineer? I'm not even young and I played out the scene from Office Space where I calculated how long it would take me to completely run out of money and ultimately go homeless - and in San Francisco that number is not long! How can anyone afford to start a business in San Francisco?