In theory, if the owner wants to expand their building, there are schemes where the developer will compensate the tenants by offering them units in the new building at the old prices, or sometimes units elsewhere. San Francisco could potentially implement this by making it possible for the owner of a building to voluntarily convert a non-rent-controlled unit to a rent-controlled unit, allowing old buildings to be replaced by offering the tenants converted units in other buildings (the total # of units on the market still increases). Of course nobody likes to be asked to move, but with guarantees on the price and a reasonable definition of "equivalent unit" it's a whole lot better than the Ellis Act conversions we're used to seeing.
