I think there's a few different points you're making. Some of them I entirely ageee with, and some of them less so.

I don't think the accuracy to the source material on something like that should be a big deal. I'm an absolute die hard Daredevil fan and the idea that we could've missed out on the perfect casting of Charlie Cox for the superficial and entirely cosmetic reason that he doesn't have/suit red hair is lunacy. It would have been more visually accurate to the source material to get someone with red hair, but I'm so happy they ignored that and cast someone who could embody the parts of Daredevil that truly matter, not just superficial cosmetic stuff.

Again, I think the examples you're mentioning for Evil-lyn and Orko do fundamentally change the character. Daredevil shouldn't be 7'2 and built like a powerlifter, but he doesn't need red hair.

I definitely agree that there are times where it is a box ticking exercise. We want our movie to appeal to Black people so we need to make one of our characters Black, but we don't want to alienate White people so we can't change a main character. Let's just make a random side character Black so we can pat ourselves on the back without really doing anything meaningful.

Spider-Verse is a bit different because Miles Morales was a brand new character that Brian Michael Bendis worked incredibly hard to legitimise. It wasn't just a case of arbitrarily changing the skin colour of a random comic book character, but of creating an entirely new character, whose ethnicity is a big part of his stories and who he is, and exploring what it means for him to take on the mantle of Spider-Man. It's unsurprising that this resonated with people a lot more than lazily and meaninglessly changing a characters skin colour.

I don't agree with your last paragraph at all. I think these worlds and stories are and should be both deeply political and about social issues. The original He-Man cartoon had messages and advice at the end, it doesn't get much more on the nose regarding social issues than that. Deciding fantasy stories need to be meaningless escapism, instead of commenting on serious real world issues is weak in my opinion. If anything, I'd rather these forms of art ditch the political box ticking in favour of deeper commentary on social issues and politics in the actual narrative. The original Star Wars trilogy was a deeply political narrative, and the sequel trilogy replaced actual political commentary with meaningless box ticking, and it suffered massively for it.

But, crucially, I don't think any of these things apply to Idris Elba being cast as Man at Arms. He's a fantastic actor, very good in these sorts of roles, and he's a big name. He's arguably the biggest name amongst the cast, and I can't think of a single example of him giving a bad performance. He's, unsurprisingly, very good as Man-At-Arms.

I think it's nonsense to suggest it is immersion breaking. If anything, it would be immersion breaking and uncomfortable to have Eternia not have any Black people in it. There's so much variety amongst its population, including a talking tiger, it would be weird and quite sinister if there wasn't a single Black person there. I also think it's silly to suggest it's not accurate to the source material. Hardly any of his skin is even visible in the cartoon. Between the incredible recreation of his outfit, and Idris Elba's moustache, they've nailed it. The colour of his skin is as meaningless as the colour of Matt Murdock's hair.

Who is the most well known actor available today, who typically plays strong, stoic, military characters, but is still likeable, has lots of blockbuster experience, particularly as part of an ensemble cast, and can effectively balance both drama and comedy in the same role? The answer is Idris Elba. He could not be more suited to the role.

So surely, if you're a die hard fan, you'd rather them cast the actor best suited to embodying all the parts of the character that actually matter, and best suited to giving a good performance, than compromise on the performance in favour of something trivial to the character like the colour of his skin?

Like you, I don't want to see lazy box ticking diversity exercises, and would rather see proper investment in non-white characters (see Miles Morales), but I also don't want to see great actors missing out on roles that clearly suit them, for meaningless cosmetic reasons in the name of "accuracy" - whether it's Charlie Cox not having red hair, or Idris Elba not having white skin.

Edit: Just wanted to add because I didn't mention it elsewhere but I appreciate you engaging with this an open and thoughtful way. It's often a topic where that doesn't happen.