No, just no. There is no reason and claiming "multicultural society" is bullshit. It's especially more bullshit if you've seen the artbook for the movie. There is no justifiable reason for the Major to be white. Even if it was set in the US there are Asian cops so there is still no justifiable reason.
You guys realize there are Asian people in the US, right? There are Asian cops. There are Asian highschool students. Literally any ridiculous reason you can come up with to try and justify whitewashing for American adaptations, the same exact reasons could be used to justify Asian actors being used instead.
I'm not saying they couldn't be asian, I'm saying in this instance, it doesn't really matter. You're free to have you're own opinion in it, but saying Asian people exist isn't an argument, or a point dismissing any other argument. I'm not justifying white washing, at the end of the day, it happened, everyone knows why. I'm saying in these two instances, it makes no difference to the material.
There are many justifiable reasons, at least in the context of "this is reality and this is how things work" and its entirely reasonable within both the source material and the film itself.
You're free to not like it and have issues with it. But that doesn't make that line of attack against this film in particular any less weak. No one is saying, pretending or even alluding to that Asian people don't exist outside of Asian countries, you've extrapolated that all on your own. So if you're going to be upset about words you put into the mouths of others, that's on you.
Really the only justifiable reason you need Is that it's an adaptation of a material that's being created largely to cater to a different and larger audience. Thus the studio got a popular actor to star in the role to put people in seats. So the film was built around that.
As for Death Note, it now takes place in a culturally different country, the united states, and if they're using the "White Privilege" angle for Light, then it's entirely justifiable, because it's an adaptation of the source material that's taking liberties to do something different and tell the same story from a different angle. There's also the angle of race issues in America, which could be why L is now black, to play upon said issues and the dichotomy between L and Light and the differences in their situations and positions when living in the United States.
Bottom line is: The race of these characters in their current iteration has no bearing on the story, and thus nothing is lost story wise should their races be changed, arguing otherwise is moot. In the case of Ghost in the Shell, you know very well why her race was changed. But in the context of the movie, and series in general, it's of little importance. As for Death Note, the race of the characters could now play an important part in the stories undertones and it now might add an interesting dimension to the story and a look at the division in America among races.
So no, it doesn't work in reverse.