I get why some fans wish Iron Fist or Dr. Strange had been Asian. If no one else had argued they should be, I probably would have. Old comic books have no shortage of white male heroes.
Let's be the best sort of conservatives and ask what's lost by making those characters Asian.
The quick answer: everything interesting.
1. As white people, Iron Fist and Dr. Strange find purpose in a culture that is not their own. The message: we do not have to restrict our learning to the culture associated with our race. We may find greater truths in other cultures than in our own.
2. As white people, Iron Fist and Dr. Strange are outsiders who struggle to fit into a new culture and are eventually accepted by most of its members. The message: we do not have to "keep to our own kind." We can find friends and lovers anywhere.
3. As white people, Iron Fist and Dr. Strange come from a culture where people who are superficially like them tend to rule, but they accept masters who are not white. The message: we should serve the best people of any race rather than the best people of our own.
Now, as a long-time fan of Hong Kong cinema, I would've loved having the contemporary equivalent of Bruce Lee, Michelle Yeoh, or Jackie Chan playing Iron Fist and the contemporary equivalent of Chow Yun-Fat, Anita Mui, or Maggie Cheung playing Dr. Strange.
But they would be playing Asian stereotypes. They might be playing great versions of stereotypes in movies I would watch many times, but they would be stereotypes none the less.
A more interesting change would be to make the characters black Americans. But that would also lose something. With white protagonists, the stories of Iron Fist and Dr. Strange imply great truths can be learned from people who are not white. With black protagonists, their story would imply great truths can be learned from people who are not black, an implication that would not necessarily be racist, but easily could be.
Mind you, I'm not saying Hollywood had to make the characters white men. My idea of the essence of a character is rarely defined by race or gender—the exceptions being characters like Captain America who are tied to a historical period in which a particular race and gender are logical. In the 1970s, I wished I could write a Legion of Super-Heroes story in which it was revealed that the heroes of the 30th Century had up until then appeared to be mostly white and male because that was a fad, but now the fad had passed, so they were reverting to the bodies they were born in, which would've created a Legion that was 50% female and very racially mixed. In the 1980s, I wished the British Avengers remake had cast Chow Yun-Fat and Michelle Yeoh as Steed and Peel. I love creative recasting in general—who doesn't love Samuel Jackson as Nick Fury?
But changes have consequences. You must always know what you lose when you make them.
That said, while Tilda Swinton did a fine job as the Ancient One, I still think they should've cast Michelle Yeoh.
ETA: BlackBeltJones On Iron Fist & The Ivory Issue: Is There Really A Problem?