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You nailed it. The decline has been going on for a long time.

Detroit received widespread acclaim for its leadership in the program, which attempted to turn a nine-square-mile section of the city (with 134,000 inhabitants) into a "model city." More than $400 million was spent trying to turn inner cities into shining new monuments to government planning. In short, the feds and Democratic city mayors were soon telling people where to live, what to build, and what businesses to open or close. In return, the people received cash, training, education, and health care.
The Model Cities program was a disaster for Detroit. But it did accomplish its real goal: The creation of a state-supported, Democratic political power base. The program also resulted in much higher taxes – which were easy to pitch to poor voters who didn't have to pay them. (Mayor) Cavanagh pushed a new income tax through the state legislature and a "commuter tax" on city workers.
Unfortunately, as with all socialist programs, lots of folks simply don't like being told what to do. Lots of folks don't like being plundered by the government. They don't like losing their jobs because of their race.
In Detroit, they didn't like paying new, large taxes to fund a largely black and Democratic political hegemony. And so, in 1966, more than 22,000 middle- and upper-class residents moved out of the city.
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And so, you might rightfully ask... after five years of centralized planning, higher taxes, and a fleeing population, what did the government decide to do with its grand experiment, its "Model City"? You'll never guess....
Seeing it had accomplished nothing but failure, the government endeavored to do still more. The Model City program was expanded and enlarged by 1974's Community Development Block Grant Program. Here again, politicians would decide which groups (and even individuals) would receive state funds for various "renewal" schemes. Later, Big Business was brought into the fold. In exchange for various concessions, the Big Three automakers "gave" $488 million to the city for use in still more redevelopment schemes in the mid-1990s.
What happened? Even with all of their power and all of the money, centralized planners couldn't succeed with any of their plans. Nearly all of the upper and middle class left Detroit. The poor fled, too. The Model City area lost 63% of its population and 45% of its housing units from the inception of the program through 1990.

Lot's more at the source.