Their race doesn't matter, their experience in an America that thinks race matters as to the content of your character does.
This thinking reminds me of those old movies that feature an interracial couple, usually a black man and white woman. When they reveal their intention to marry, the woman's parents tell them that they (the parents) don't mind, but "think of what your children will face". It's the rest of America that wouldn't accept a black actor playing Cap or Starks. This type of thinking is why it has taken so long to get a "Black Panther", because the people in charge believed that it wouldn't be accepted.
It would require they be written with some level of plausibility, in which case you can not ignore the changes to their story required by changing their race.
You can write them as unicorns from space if you want, but if you don't make that plausible within the context of the story it is going to be a bad story.
You don't lose anything, but you do change it.
Social equality does not mean sameness. Writing black, white, asian, etc. characters the same is disingenuous and almost always defaults to writing them as white. Race may not exist, but racism does and societal structural racism especially can not be ignored.
With respect to the bolded text, that is correct, you don't lose anything. This is part of the point I have been trying to make from the start. There is no substantive reason Cap and Stark could not be played by black actors just as there is no substantive reason both
have to be played by white actors. The only argument against them being portrayed by black actors is the belief that "people wouldn't accept it, which means that the only argument against it is racism.
What you refer to as "writing them the same" is really a reference to writing a character as race neutral. Cap and Stark are both written as race neutral, meaning the character's race has no significant meaning or impact on the character's story or who the character is. The only reason the characters appear white is because that is the how their creators happened to have chosen to make them look. In the case of the movies, Cap and Stark are white only because the actors chosen to play them are white. But there is nothing written into either character that
requires them to be portrayed by white actors.
But when your default setting is white, meaning that you see every character played by a white actor as in-universe white, then "race neutral" means "white". Nick Fury was written as race neutral, believe it or not, that's why it was so easy to change his presentation in the comics to black, and to have him then portrayed by a black actor in the films. Sam Jackson is black without question, but Nick Fury is not, or you might say, he is the same race as whichever actor is playing him.
It's done all the time. Ever see a Denzel Washington movie? How about a Will Smith movie? Many of the characters Denzel and Will play aren't black in-universe, though I imagine for some people, even fans, they aren't able to separate skin color from character either.
The problem isn't the race of the actor in most cases, the problem is with the observer. Sometimes the observer has to be dragged kicking and screaming into the 21st century.
You're not kidding. Someone is doing some serious backtracking here.