This is a tricky thing. For one, you can quite legitimately ask where to draw the line - if you're fine with someone being brunette instead of blonde, why aren't you fine with dark skin instead of light skin, for example. Where are you drawing the line? Physiological differences between humans are fact - grouping them into "races" is largely a construct. So who gets to draw the circles and say who's in and who's out, and why? Isn't suspension of disbelief exactly for such circumstances, putting performance over accuracy?
Then again, there are absolutely cases where things like skin color MATTER. Some are fairly clear-cut - a white MLK would take so much away it wouldn't work. Others are more debatable - what about a black Hamlet? Not historically accurate of course, but is that important to the role? The edge cases are very difficult to decide, because - again - a lot of the distinctions are given fairly arbitrary weight. A white Jesus, when he's a Levantine Jew? Why does that work more than a black Hamlet - and should it? And so on.
Which also brings up a whole slew of historical biases. Because race as we understand it is a fairly recent concept, we tend to project our understanding back onto historical settings. Ancient Rome, for example, was very likely quite racially diverse. Why aren't black people mentioned a lot? Because people then probably didn't care as much about skin color as we tend to think they would have. We think of Cleopatra as "Egyptian" - she was a ruler of Egypt, but her lineage is straight from Greece, not from Africa. We tend to forget that nation states, too, are a very recent concept.
All this makes it really tricky to find the right angle for representations both fictional and historic. You want to balance modern-day sensibilities, aesthetics and iconography, historicity, and so on - often an impossible task to perfectly solve. So where do you skew things, how, and why? Is a skew towards more diversity in casting "more important" than historical accuracy? If so, why? If not, why not? Is historical accuracy "more important" than the aesthetics of the performance? How do you even determine that?
It's very complicated, and can't JUST be reduced to "we do this or Twitter will have our heads". Not that there isn't plenty of pandering and ham-fisted shoehorning going around, that should be rightly criticized. But it isn't always as simple as people make it out to be, on any side of the issue.
For me personally, the key is always rooted in the narrative above all else. What does the PLOT demand? Would this not work if a character is a different gender, race, whatever? If it changes, how does it change? What does this mean for the rest of the plot, all the contingencies, the world-building? There's no perfect answer, and no simple scale to go by.
The WoT situation in Two Rivers is kind of a good example. Canonically it's supposed to be the remnant of a previous empire - but how homogenous was that empire? Many centuries are a long time, and unless there's serious impediment (like an ocean) people WILL mix over time. Which might mean a new homogeneity, or might mean more diversity. But what does the PLOT say? These are "one people", with a certain pride and history - okay, but "people" doesn't equal "race", and you could have all sorts of people only vaguely similar with considerable leeway. What actually MATTERS for the plot? That Rand stands out, and that he belongs to a group that's not just geographically but also CULTURALLY isolated - that's often a stronger isolating factor than geography (see e.g. with hermetic religious communities like the Amish). So it makes sense to pay attention to race there, doesn't it? Yes and no. Because cultural isolation also means culture is more important than race - see e.g. the Dune series where culture as an identifier has practically supplanted race entirely. So how do you deal with that in a visual medium like film? Do you double down on visual markers and extend them to physiology? Or do you stick to cultural identifiers, and simply visualize them (e.g. distinct clothing, a special color, etc.)? In Rand's case I think race makes sense because he's isolated from the cultural aspects - marking him as an Aiel culturally doesn't work if he grows up in a different culture. It has to be something inherent, like appearance, if he's to be marked without actively participating in that marking. The plot has an actual tether to appearance here, and it makes sense to pay attention to it as a result. But that's not always the case, and each instance comes with its own expression, contingencies, and often complex considerations about how to represent it in both visuals and narrative.