New construction sometimes doesn’t even help, when developers knocks down an old affordable 12 unit apartment building and build a luxury 36 unit building, you’ve created -12 units of affordable housing.
The argument I hear against this is that the 36 people who move into the luxury apartments moved from somewhere, and so 36 other apartments become available. The reduced demand for the vacated apartments then drives their prices down.
Of course, housing as a market is super distorted for a bunch of reasons so this effect is muddled. But I think it would be a net negative to fully disregard supply and demand in a market-based economy and preserve 12 affordable units in favor of 36 luxury ones.