[slg]

>all these would never filter down to affordable levels.

Another factor contributing to the same thing is that cities aren't closed systems. Building new units at the top of the market doesn't mean that everyone in the city will move up one rung in quality leaving the cheapest unit now vacant. Many cities will have a latent demand for housing that is being suppressed by high prices. When that exists, someone from outside the city will move to the city to occupy the new construction or the newly vacated units at the top of the market. All that latent demand needs to be satisfied before the benefit of the new construction will trickle down to the bottom of the market.