> Sometimes, you cannot build more housing, or denser housing ... If you turned San Francisco into Manhattan, and still had unquenched demand for housing while still maximizing every square meter of three dimensional space within desired geo bounds, what then?

Using Manhattan as your example of maximum possible density is... odd. Kowloon City [1] had much higher density and had the disadvantage of poverty.

I'm not saying that we should turn all residential areas into facsimiles of Kowloon City, but there is a ton of space between what currently exists - even in Manhattan - and that. I'm deeply skeptical that any place in the US or Europe has reached the limits of building more housing and is forced to look at other options.

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1. https://en.wikipedia.org/wiki/Kowloon_Walled_City

"Build baby build" is a pretty good silver bullet - assuming the city is willing to keep up with infrastructure spending to support the denser housing (absolutely possible economically, but politically difficult) _and_ assuming the local populace doesn't throw whoever advocates for it out of office. Too often new construction is killed by NIMBYism (though, it seems just as often new construction blames NIMBYism for legitimate complaints about land compensation)... if you live in a city and treasure your quiet off the beaten path neighborhood... too bad.

>Sometimes, you cannot build more housing, or denser housing

A hypothetical possibility that doesn't hold true for any city that has enacted rent controls.

"Build baby build" is a silver bullet.

In New York City the subway is running at capacity. Building housing isn't enough, you also need infrastructure.

Brooklyn resident here. Plenty of room on the subway during my morning commute. Wish they would build more condos here so I could enter the market.

Manhattan uws has lower rents than comparable places in SF in my experience.