Wait, if racially diverse is the extreme... what's the medium?
The only reason I think it extends naturally is because that's what I see time and time again. Look at the X-Men or Avengers. Or any big popular bit of entertainment set in the modern day. Lots of diversity, but the Matrix isn't *about* cultural differences, any more than a story where people from different jobs work together is about their differences, but *all* differences make the story deeper and better.
Now what you're saying is true for the opposite, stories that are about cultural differences and inequality are often nearly all white or all black (or all whatever ethnicity it's about), because that fits the story, which is about exclusion. Mad Men. Roots. Inclusion story -> Inclusion characters. Exclusion story -> Exclusion characters.
On your example, we're not talking about starring roles, we're talking about inclusion as heroes. Rhodey being included, as an equal superhero no less is an example of diversity enhancing a story. You have two guys, good friends, different as night and day - on top of that, one of them is black and one is white, making them even more
different. See how subtle that is? Then they both end up being armored heroes fighting back to back in different colored suits with different weapons. Its a simple little touch that adds a thousand nuances to the story, which is not about race. Black Fury does this same thing to an even higher level with even more nuances.
Now, what is your basis for thinking differences in ethnicity and gender are less
nuanced or complex than differences in jobs and clothing?