Comments on the Guardian website claim the vacant space problem is exaggerated.
I haven’t been in SF for about ten years personally.
Only in the sense that there is always a new sucker who thinks SF is "great" and they can make a go of it where others have failed. The turnover is extraordinarily high.
A lot of San Francisco's character disappeared during the dot-com boom in 1999-2000, when flaky Internet companies were willing to use their investors' money to pay exorbitant rents to take over retail and restaurant spaces at a time when available office space was almost non-existent. After that, rent levels stabilized at a high plateau and only big risk-takers and financially strong chain stores could afford to operate (at a loss) anywhere near downtown.
It's a twenty-year-old problem that reaching its zenith now.