>building lots of lower rent units will bring down the rents in the luxury segment
Sure. Wealthy people can lower their standards and take housing meant for the class below; people who are struggling can't exactly raise theirs, but they do group into larger/more crowded households to bid upmarket. When there are not enough small apartments, for example, people find several other working housemates for single-"family" homes that are then no longer available to 1- or 2-income middle class families. I suspect the pressure is stronger in the downmarket direction but it absolutely cuts both ways. (And homes that are too upmarket sometimes don't act as housing at all).
>What you're arguing for is a removal of the poor from certain parts of the city they currently exist in
Nope. With sufficient heights, the problem is addressable through infill development on commercial/industrial properties (ideally replacing existing uses on the lower floors) and upzoning to permit mid-rises in predominately owner-occupier single-family house zones.
>We don't need an ideal world or a revolution to build and maintain affordable housing.
The Bay Area's housing deficit over 2012-2017 is conservatively 120,000 homes based on 400,000 new jobs, 60,000 new homes, and an average household size of 2.3 (many household members are not workers). [0] The average cost to build an affordable housing unit in California (so an underestimate for the Bay Area) is $332,000. [1] So $39 billion is a lower bound to walk balk just 5 years of a problem decades in the making. Where's the money going to come from? Not property taxes: prop 13 repeal would be a revolution in itself. Not federal subsidies: do you think the Trump administration wants to subsidize coasties? Not tech companies: there's always some Bay Area municipality that's happy to keep the free tax revenue and soaring housing demand (particularly for owner-occupier areas) local. San Francisco spends tens of millions on affordable housing when what's needed is tens of billions. Even the state of California spends only hundreds of millions.
So far the best anyone has come up with is (effective) taxes on market-rate development. To get the numbers required, we're going to need a hell of a lot more market-rate development to tax.
[0] https://www.bloomberg.com/news/articles/2017-06-23/why-can-t...
[1] https://www.mercurynews.com/2017/08/16/walters-bills-would-m...