Key graf:<p>The theory goes: Vacant commercial towers in San Francisco will likely never again reach capacity, but they could be converted to student housing by public universities, which are exempt from much of the red tape that otherwise stymies development. These schools also receive substantial tax breaks, making the cost of such projects “pencil out.” Large cohorts of new, young residents would not only make use of the buildings themselves but also reinvigorate Downtown by supporting local restaurants, bars and other small businesses. The idea might not be a silver bullet for saving Downtown, but it’s a possibility that makes other ideas floated over the last year feel small.