I mean, one of Heimdallr's descriptions is "the whitest of the gods", and has his origin in Northern Europe, so I don't know why typically depicting him as white should be a shock.
Hypothetical: MCU starts introducing New Warriors characters at some point in the future, would you be OK with it if they cast a white woman as Hummingbird? Do you imagine it would go without upset and outrage from certain corners of the internet? In case you aren't familiar with that character, in the comics she's a Mexican girl and also probably an incarnation of Huītzilōpōchtli (the Aztec deity of war, sun, human sacrifice, and the patron of the city of Tenochtitlan).
This is why this is such an issue. If you flip the situation around where white men are getting cast for X that traditionally had POC/women/LBGTQ, there can and has been an epic shitstorm. Cast people because they are POC/women/LBGTQ to replace white men and women though? Not a peep despite the fact that it is 100% just as racist/sexist/whatever as the other way around. I don't know about everybody else but it's the sheer hypocrisy that bothers me the most whenever this happens.
...and you'll notice most of the times you see significant bitching, it involves casting pre-established characters in a way that doesn't at all resemble their existing descriptions. Hell, there wasn't even much complaint about black Nick Fury, mostly because there was an extant black Nick Fury (albeit from a different version of earth than most of the MCU is set in).
Plus, it's mother******* Samuel L Jackson. I'm pretty sure nearly nobody is going to complain about that particular casting. Anybody who has ever heard of the guy knew he won that role on his acting chops and he would do an excellent job, which he did. He wasn't put in because "hey, let's make Nick Fury black as a publicity stunt." Which is what people are actually complaining about. People getting cast because of their race, gender, and/or sexual orientation rather than the only thing that should actually count, their
merit at the job is the problem for the vast majority of people who complain about this except a tiny microscopic minority of racist/sexist/whatever assholes.
Robins either age out of the role and take on a new identity to get out of Batman's shadow (Nightwing, Red Robin, Spoiler/Batgirl), or they...don't survive long enough to get there (such as being beaten to death with a crowbar by the Joker).
On the note of Spoiler, her entire tenure as Robin was basically everybody telling her "You suck because you are a girl" and her repeatedly proving it until she faked her death and switched to becoming Spoiler, which was her taking on her own identity rather than taking another's like I suggested doing earlier and when everybody in universe and out started giving her much better reception. It's almost as if taking a traditionally male character and slotting a female in their place for publicy's sake is almost always an extremely terrible idea that rarely ever results in good works of fiction.
Right, yes-- and when it wasn't a white dude, people moaned more.
I don't recall anyone having any issue with John Stewart when he was the DCAU Green Lantern.
Yes, people tend to moan when a traditionally white character is portrayed by another race/gender/LGBTQ, but that's correlation, not causation for the vast majority of people. For most people they're moaning because of the issues that nearly always accompany this kind of change in portrayal. Like blatant publicity stunts, it was probably forced in by executives, the writing for them tends to be godawful because the writers never wanted to actually put them in so they're looking to torpedo the character so they can get the go ahead to put the originals back, the popularity of the original character, hypocrisy on the part of the people both making this decision and the people supporting it...
In short, people tend to moan about this because all too often it makes whatever media awful.
For an example of this, see Miles Morales, who showed both how to do this wrong and how to do it right. In the Ultimate Marvel Universe Peter Parker gets killed off, emotional ending for a hero, all that jazz. Then there's fanfare about there being a new "Black Spiderman!" all over the net for weeks. Nothing about who he was, his motivations, anything, "he's black" is all that is shown. Also half Hispanic, but even that was swallowed by "HE'S BLACK!!!" This irritated people long before the character debuted. The first several issues were even about the fact that his attempts to emulate Peter while wearing the same costume are pretty insulting to the dead kid's memory and how uncertain Miles was about filling his shoes. Which showed Marvel was perfectly aware of how badly they were botching this. It took years of work, changing his outfit, and eventually transferring Miles over to the main comics before the character was redeemed, when it wouldn't have been tough to just do him correctly in the first place.
Cut to the Spider-Man PS4 game. A bit before Into the Multiverse made Miles a main protagonist on the big screen in his own right. Peter is the main protagonist and stays alive. Miles has a few scenes here and there that help establish him, he forms a relationship with Peter as Spider-Man, Miles shows his own cleverness in NOT annoying gameplay sections, and the game ends with Miles getting spider powers and showing that fact to Peter, which leads up to him getting his own game.
Miles spends the game as a supporting character long before he gets spider powers of his own. Miles isn't a publicity stunt, in fact the game's advertising had very little about Miles prior. Miles is portrayed as a human being and not a caricature. Miles isn't upstaging Peter nor does Peter have to die or step down for Miles to take center stage... He pretty much avoids every pitfall he fell into in his origins or other similar characters did. Into the Multiverse only cemented Miles as a worthwhile Spider-Man.
It's all because of one simple reason: Miles Morales being black had NOTHING whatsoever to do with his character portrayal or why he was included in the latter two cases. Miles was just there to fulfill a role in the story. Mile's race wasn't treated as significant at all by either the narrative or the advertising, which is how it should be. Outside of comedy people of color/women/LGBTQ should be portrayed as perfectly normal by the narrative and have little to no fanfare outside the media they are in. As well as not put there just to have them, they need to be vital to the narrative on their own merits with those merits having nothing to do with which diversity check boxes they mark off.
In short, if you can tell a diversity character exists solely for the sake of diversity they are being done wrong.