There’s something that I think often gets overlooked about the replacement of old buildings with new ones, and I think it’s pretty important. The lowest tier of housing in a given city is generally accessible to poor people in ways that new construction, or even properly maintained older buildings never will be. Slumlords operate at this level, and while they suck for many other reasons, they are often willing to rent to anyone who can deliver the rent money approximately on time. No background checks, no credit checks, no income requirements, just cash on the barrelhead the first of the month. That’s a hugely valuable service to the poor; no matter how bad the housing might be it still beats living on the street.

There are a surprising number of homeless people who live off of their disability benefits. If they could find such an arrangement as described above, they wouldn’t be homeless anymore. But landlords have no incentive to take that risk with them, and they won’t pass any of the income screening processes that are baked in to the apartment rental process today. As the worst of our housing stock disappears, the fraction of the overall housing market that remains accessible to people like that becomes ever smaller.

Worse, it’s a ratcheting effect: it only works in one direction. Sure, as the new high end units go up, the previously acceptable ones look less desirable, but it takes a long time for a unit to reach the bottom rung. Plus, even if it’s now the least desirable building in town, if it belongs to a landlord unaccustomed that to renting to the poorest segment of society that can technically afford housing, it’s likely that landlord will not want to change their requirements and policies to begin renting to them. The landlord is likely to preferred to sell the property instead, which greatly increases the chances it will be redeveloped into luxury housing, and so on.

But that is not actually what happens in the cities like SF. The lowest tiers of housing are not affordable. The worst apartment I've ever lived in my life (including Soviet-built flats) was in SF - a crappy 1906 victorian (or whatever you call these ugly hovels SF is full of) subdivided into 6 units; drafty, not updated in ages, no dishwasher or laundry, etc. It was also the 2nd or 3rd most expensive rental per sqft in my life (the only other ones were also in SFBA). I took it because I didn't even realize apartments SO crappy can exist, e.g. I didn't realize there was no dishwasher until after moving in... and the whole neighborhood was like that. Normally, that would all be "lower tier" housing. In SF, it was $3k/mo in 2014 and rented to either techies, or roommates (sometimes 2 unrelated people per bedroom). If someone built luxury towers in the same neighborhood, the techies would have moved there and actually made the lower tier of housing somewhat cheaper for others.