There are ways to return cities to vitality and efficiency, but it takes a lot of planning, and it varies somewhat, based on the city. Here are some highlights.
1) Of all places, Detroit had a good idea. A city in an advanced state of decay, with large areas of abandoned buildings, they got the simple notion of bulldozing huge tracts of decay, and planting forest-like parks in their place. Positively brilliant, if not for the best reasons.
Why? To start with, the curve for redevelopment strongly increases with available area. That is, you can only put so much on the lot of a single building, with existing infrastructure. But if you have an entire square block to develop, you can do a lot more, from below the ground up.
And, if you have several square blocks, you can have almost completely new everything. Good water and sewer, good electrical and communications, even better transportation and your own residential neighborhood and retailers. Together it is worth far more than its individual parts.
But while you are waiting, planting a forest there is the fastest and cheapest way to clean the ground of toxins and poisons. Just periodically cut everything down and haul it off for waste disposal, then plant anew.
2) “Rolling gentrification and maintenance”. Subdivide the city into districts, and intensively gentrify and maintain each district in turn. This means doing little work in quality areas, and most work in the areas that need it the most. It creates a cycle that prevents parts of the city from becoming run down and staying that way.
They did this in the South Bronx. The Bronx had blocks of burned out and abandoned buildings. Almost all are gone now. Single family homes have replaced most of the old brownstones.
They are starting to clean up Harlem. The problem is what to do with the current residents.