Of course you could probably find some measures for which your statement is true, but here's a graph of first one that occurred to me where I thought I could get the data, which is the last 20 years of Oakland County median household income [[deflated by the CPI) vs Wayne County median income similarly deflated. A couple things stand out.
1) Real incomes in both counties have dropped.
2) There's not a huge difference in the percentage drop, but it looks like Oakland dropped a bit more.
3) Oakland is still relatively very well off.
So I don't think it is at all obvious that "Oakland boomed while the other counties withered", although I would hardly say it is in some kind of massive decline either.
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