...led to wholesale elimination (through upgrading or demolition) by the 1970s.

That's the good news. The bad news is that the bottom rung of the cheapest housing simply disappeared, and the cheapest rents with it. With the rise of genteel suburbia, housing standards rose considerably, enforced by local code police and homeowner associations. With the bottom rung of housing gone, there was no longer a cheap bottom rung to choose, and competition for the remaining affordable housing stock soared.

The Tax Reform Act of 1986 changed the tax rules for owning rental property, and increased t he importance of positive cash flow in rental investments. This shook out some weekend landlords and created a trand toward professional landlording, in turn promoting higher rent structures.

Energy prices are an emerging impediment to a minimum wage standard of living. Many low-wage workers are being forced to live further and further from their jobs, to find affordable housing. Add in the rising cost of commuting, and there is an emerging crisis in the works.