Earlier quoted context omitted.
...the houses would almost all be built for the very well off. The amount of houses you'd have to build before you started constructing truly affordable housing is immense. This is a common mistake. In reality, all added supply brings down price levels across the board.
Actual market dynamics are complicated. There are so many effects that happen when housing is built. New housing in SF is being sold to out-of-towners who don't actually live there. Building housing into a downtrodden neighborhood can improve it significantly and actually increase values, etc.
"Market-rate housing probably has contributed to gentrification in many cities in the past. But the data shows clearly that, in San Francisco in 2015, market-rate housing development is not accommodating upper-income movers who would otherwise be somehow shut out of the city's housing market. On the contrary, the overwhelming majority of wealthy new residents do not live in new housing, and conversely, the vast majority of new market rate construction creates housing opportunities for existing city residents. For this reason, we find no reason to believe that either a temporary moratorium, or an indefinite prohibition, of market rate housing will reduce the number of upper-income residents in the Mission, or slow the process of gentrification that the neighborhood has been experiencing."