Let's say I'm a developer and there's unmet demand at all price points. Obviously I build the highest-margin product until I've satisfied that demand. Then I move on to the second-highest-margin product until I've satisfied that demand. Anywhere that demand across the spectrum dramatically outstrips production capacity (e.g., anywhere in the Bay Area, where NIMBYism has made it nigh-impossible to build anything) what…
It'd be perfectly reasonable to mandate that for one high value luxury house, a developer has to build a certain number of affordable homes. I guess it'd be the nimbys that would try and prohibit such legislation.
The only thing that should happen, if the goal is to lower prices, is increase supply and/or lower demand. The supply can be increased by simply changing zoning to allow for much denser residences.