I wouldn't read too far into that stat. Chicago has a large poverty-stricken population that is most likely single-parent households and receiving government assistance compared to the overall American black population.

I think we all forget as recent as 1950 most American cities were majority white. Having a black/white/hispanic/Asian majority doesn't make a city great.

Does Detroit barely having any white people make it suck?

The middle class black residents aren't being replaced. The ones you see on these lists are just the ones that were left behind. And, it could very well be the case that the heat map from above is showing the decline in economic status of black Chicagoans and not just the flight of them.

Most of Chicago's black working class and middle class would have been employed in local and state government jobs (teachers, city services, emergency services, etc.), or manufacturing jobs. We all know what's happening with manufacturing, but there was also a big trend of privatizing city services, and closing public schools in favor of charters. And the black families in professional worker households (doctors, lawyers) would've mainly serviced those black middle class workers.

We didn't forget that; it's not really true. The biggest American cities have always been diverse.

Obviously, Detroit is not a very cosmopolitan place. Obviously, the lack of diversity is a key factor in it not being cosmopolitan.