I agree that it sucks if this sort of casting decision was made based purely because of direction from the studio. Marvel has had a good track record of honoring the source material pretty well.
There's also Tilda Swinton. The woman is an amazing actor! And I think people will probably be impressed by her portrayal of the character. She has put together so many consistently strong performances, and I don't have any reason to doubt her in this case.
As for Asian actors in key roles? I think if there's a fairly positive role for an Asian actor to play, and they can find the right actor? I can see them going in that direction. Funny thing though about one Marvel related film that featured a lot of Asian actors - The Wolverine. Granted, that story took place in Japan, and it called for a largely Japanese cast. But one of the key villains, Viper, had her ethnicity changed for the movie. Viper was an Asian woman in the comics, but depicted as a Russian in the movie.