The factors that contribute to urban decay are many and they are unique to each city. Cities that developed centuries ago may have built up around industry, on travel routes or at the best places for seaports, with housing, shopping, hospitals and other services patched in to support the growing populations. There were patchworks of farmland, manufacturing plants and the housing that supported all sorts of people.
As buildings aged, it was easier and cheaper to move outward and to build new structures than to refurbish the old. The value and asking price of the abandoned properties allowed poorer and newer arrivals to move in for cheap rents. Ghettos formed, and ghettos involve all races, ethnicities and income levels: people grouped into neighborhoods of their own kind, welcoming some and using violence to reject others.
As people moved to the suburbs, more people from all levels of society moved into the cities to replace them. The cities began to develop much more along lines of planning and increasing municipal and government oversight and control. Huge properties of hundreds of acres were subdivided and sold off, but under the principles of planning and zoning.
As a result, parts of cities go through periods of growth and decline based on many factors, but especially because of the age, zoning and size of the buildings and structures. Then there may be lack of interest, investment or ability to keep the structures from deteriorating from overuse or abandonment. Finally a growing population and economy is needed to make it viable for the deteriorating or unused structures to be refurbished or replaced.
Thus, a part of a city might become solely industrial while another part was strictly for housing or shopping or finance. If an industry abandoned their plant, then that area would become blighted unless it was converted for some other industry or use. In San Francisco, California, Hunter’s Point Naval Shipyard is a buzzing hive of housing and smaller business. Base closures in Sacramento, California did not stop United Parcel Service and other businesses from turning the facilities into major business hubs.
Some urban centers, like Flint Michigan, grew up and died around a single industry without ever recovering. Some cities provide support for the farming or steel industry for an entire region. Others are based in fishing or sea ports. Others grew up around the film industry or major military installations. Single industry cities and towns can go through growth or decline based on the fortunes or failures of those single industries.
Urban decay is being staved of in parts of California when over built areas have industrial units and commercial units that are made available to smaller businesses at reduced rents. Even small churches are taking over industrial units that would have been prohibitively costly in a booming economy. But many unfinished and never occupied units exist and are deteriorating from lack of occupancy that has gone on for years.
The health and life of an urban structure is like the human body in the sense that a stable and healthy economy is the lifeblood of the city. When the buildings are kept up and maintained, the majority of residents are working, the tax coffers are filled and the services and planning are active, then decay will not happen, except as specific and older neighborhoods decline.
When there is poor employment, declining population, declining industry and a declining tax base, urban structures will, of course, decline and the parts of those human structures will decay, just as human tissue that has no nutrients will decay.