You have masterfully identified correlation, without a hint of establishing causation.
Detroit's affluence came about because of the voracious demand for automobiles by the rest of the nation. It was this nearly insatiable demand that led to the rapid growth of the automobile industry, and with rapid growth comes excess. Unionization of auto workers began because the workers felt they weren't getting a big enough piece of that pie, and at the time there was indeed plenty to go around. The key point, however, is that the wealth came before the unions did.
As the market became saturated, the crunch began to set in. Detroit was already having financial crises in the 1950s and 1960s. The unions basically mortgaged their current wages on the future of their industry. In so doing, they set it up for inevitable failure. The death blow came in the 1970s and 1980s when foreign auto makers, primarily Japanese, were selling better quality automobiles for a fraction of the price of their American counterparts.
Protectionist policies blunted some of the effect of this economic upheaval at the expense of other industries, but even so the Japanese started producing automobiles domestically in the late 80s and early 90s, and continued to eat Detroit's lunch. At this point, cars were a mature industry, and the union wages which were predicated upon perpetual economic growth were not sustainable.
Indeed, the slow break up of unions is correlated with negative economic changes. But once again, the wealth began to disappear before the unions did. The government only exacerbated this problem by maintaining high tax rates, which under rapid growth were affordable, but under stagnation became onerous. People left in droves, and those who remained had little money to tax.
Finally, the standard of living in the United States is at an all time high. Even Detroit, a city that by any reasonable economic assessment should have been a complete and total wasteland by the end of the 80s, has limped along thanks to the nearly unfathomable explosion of wealth in other parts of the country. Hell, there are still auto workers making good wages, especially for being in such a mature and automated industry, but most of them are not in Detroit any more.
Detroit's government and the auto workers' unions killed the goose that laid the golden egg. The city's recent bankruptcy, predicated largely by the government's inability to pay ex-workers' pensions, ought to stand as a clear monument to the folly of spreading today's wealth around at the expense of tomorrow's.