Your underlying logic of applying color/gender blindness is sound and makes sense.
In a perfect "all things being equal" scenario.
But when you look at representation of non-white, and/or non-male principal characters among most forms of media and entertainment, you will either recognize that the playing field is not equal, or you won't. If you believe that the playing field is equal, then your position is understandable, but possibly lamentable. If, however, you recognize that the field is not equal, then you simply cannot expect to apply your logic and come to a result where the field evens itself. Especially when you consider the systemic factors* that have created the uneven field in the first place.
In order to create the even field, where your logic can be properly applied, you must, for a time, re-work the system and apply un-equal forces opposite the current system in order to create a better balance.
So, you can change Valkyrie in the Thor movie to a woman of color because 9/10ths of the other principle actors remain Caucasian. Whereas, in this case, changing an Asian character to a Caucasian actor most likely eliminates the ONE Asian character from the principle cast. Or, at the very least, that one character is most likely a significant percentage of Asian characters in the movie. In losing this one Caucasian actor, though, the skin color will still most likely be more than amply represented.
* one of these factors in general has to do with investment safety, which is why producers may cast Emma Stone as a half-Asian woman or Scarlett Johannson in a remake of an anime film.
** Side note: This seems to imply that you feel that there may be no Asian ACTORS who might be able to play that CHARACTER, which may be perceived to be another one of those slippery slopes I feel you tend to frequently stand near.
