So, having Starks and Rogers played by black actors would require that their characters be written as "black" Americans? Starks and Rogers played by white actors aren't required to be "white", are they, unless you think that being super smart, having courage and integrity, being rich (in the case of Starks) are characteristics of being "white". None of those, even being rich, are "racial" characteristics.
No, we think being given combat training within three years of enlistment during World War II, getting a bank loan to start a company in the 1930s, and not being redlined and force to live in underprivileged neighborhoods are characteristics of being white in America.
There's a tendency in our culture to treat straight white christian males as the default form of humanity, seeing that people are defined by how they differ from that baseline, but that's not the case. Whiteness is not blankness. Being a white man in America is to have certain benefits conferred upon you (privileges, if you like), above and beyond what others receive, and that shouldn't be ignored. Just swapping in a black (or, for that matter, female) actor as Steve Rogers or Tony Stark as-written in the last two-dozen or so movies would be nothing less than a whitewash (if you'll pardon the term), erasing the very real history of racism that not only would prevent such people from living those lives, but prevented them from being written as black in the first place.
Now, as we've said, it's possible to rewrite those characters to fit in with how the world would've treated them in their backstory, but there would be major changes that would affect who they are in a fundamental way. Steve Rogers grew up in the 1940s, where opportunities he took advantage of as-written were flatly denied to black men. Tony Stark is a character who is explicitly the product of a privileged legacy that could not plausibly have come about when it did without making Howard the hero and Tony the ancillary character. Bruce Wayne has the same problem. Sure, wealthy industrialist is a stock character, except, again, that's treating whiteness as unremarkable. Wealth
white industrialist, even wealthy asian industrialist, are stock characters, but do you know how many black billionaires there are in the world?
Ten. Out of over two thousand. Only three of those are American; Oprah Winfrey, Michael Jordan, and a venture capitalist who is, apparently, very good at his job. None of them are inventors, or tech magnates, or captains of industry in their own right, never mind the children of such people. Because America didn't allow people like black Howard Stark to happen. It'd only barely allow him to happen today.
And, as has also been stated, those were probably the worst two characters to pick as examples of straight white males where you could swap out their demographics without affecting their stories. Peter Parker, Clint Barton, Bruce Banner, any given Asgardian, Peter Quill, Scott Lang, Hank Pim, Stephen Strange, Phil Coulson, take your pick. Over on the DC side, Clark Kent is practically begging to be anything that isn't a visible example of "legacy" Americana. He's an immigrant, for God's sake, maybe it'd help if he looked like a real-world first-generation American and not like he just walked off the
Mayflower.
So far, the only reasons you've given as to why you think Rogers and Stark cannot be played by black actors is essentially because if the actors are black this would require that the characters be written as "black" and that would, apparently, have to differ so much from a white actor, that it wouldn't work. Do I need to point out here that it is very likely that perhaps your own personal beliefs about black stereotypes are at work here. If not, please explain to me why not.
You're the one who gave examples of why the Black Panther being black was integral to his character and story that didn't have anything to do with racial stereotypes, just how being played by a white man with the same script would recontextualize him. Just apply the same form of reasoning to Steve Rogers and Tony Stark.
If you were to change him to a white man with no other changes, BP suddenly morphs into a Tarzan-like figure, which would be a much less compelling story to the point of maybe being offensive.
If you change all the other characters to white, then you lose the weight behind a story of a black African country which has never been colonized and which has been disguised as a poor third world nation. Its different with most other Marvel characters, Tony Stark's race is not an integral part of his story. Steve Roger's race is not an integral part of his story, so yes, its easy to change the race of those characters.
You don't see how these issues apply just as much to pretending racism didn't exist in 20th century America? Or, even worse, ignoring that racism existed entirely?
You know what just replacing Chris Evans with a black actor does with the Captain America script? It tells us that segregation in the Army was justified, because black soldiers actually
didn't fight as well as white ones, because the one that did was integrated. The movie would make it clear that there was one, and only one, black soldier who was able to rise through the ranks and lead the white fighting men. If segregation can't keep Captain America down, then all the other guys who look like him really
aren't good enough. If racist business practices can't keep a black Howard Stark from becoming one of the richest men alive before anyone ever heard of Rosa Parks, then there must be a reason why there are only ten black billionaires in the world and not hundreds of them that isn't "centuries of white-promoting power structures designed to make it impossible for minorities to succeed."
Changing these characters' races without taking into account how the world around them would treat them differently is the worst kind of tokenism, because it says that structural discrimination and historical racism aren't real, and that the reason for racial inequalities in society is simply that it takes the best black man in the world to match any white man who's just above-average, at most. Tony and Steve would be "one of the good ones," which implies that most black people must be one of the bad ones.
I find that notion repellant. And I am now terrified about hitting "post," because I know someone is going to take a sentence of this out of context and accuse me of holding or advocating that belief. In fact, to be clear, I'm not happy that Captain America and Iron Man have backstories built upon their whiteness. I would not have a problem reimagining them as black, or asian, or Native American, or women, or anything, so long as they
were reimagined, taking into account to circumstances that real people with those backgrounds have to face in their lives. My point was simply that it wouldn't make any sense to simply recast those characters as nonwhite without revamping their entire story, in the same way that you were saying that a hypothetical "White Panther" would have to be entirely different from the movie we just watched.