A recent article in the San Francisco Chronicle (Sarah Ravani, "Tenants evicted as low-income units take shape," Sunday September 1, 2019) notes that even when cities promote low-income housing affordability for some is sacrificed due to the lack of a housing policy that puts affordability first.
San Francisco would not be in the position of seeing its affordable units constantly sacrificed for luxury housing or "low-income" housing if it had a distinct and defined policy for housing construction. But in the context of the world we find San Francisco is not alone.
In data produced by a number of sources including Dr. Yolande Barnes of the Bartlet Real Estate Institute of the University College London, Financial Times writer Judith Evans (Biggest Cities start to shrink as inner areas empty out," 26 August 2019) found that most major cities in the world (especially the USA and Europe) are experiencing a decline in population as luxury units replace affordable housing and the trend of vacation rentals ramps up.
Companies like Airbnb are a motivating factor but the lack of defined housing policy and funds for affordable housing are at the center of the problem.