What are the chances of Miles starring in the next Spidey film (IOW, no Peter at all)? I wouldn't say impossible, but probably close to minuscule. Also, I was talking about supporting casts (for live-action purposes, Wally is part of Barry's). Starting superheroes are a different story, though, since we have quite a few legacy characters that fill the bill. In those cases, it would make damn good sense to use them before changing the racial identity of a character.
There are a lot of white actors just as good, if not better, than those two. WB wanted a prominent black face and that's why they were changed (and, in the case of Smith, he's a box office draw).In the case of Will Smith and Laurence Fishburne being cast, it was probably just the case that they are two really fantastic actors.
Nobody cares beyond our hobby, though. If Wally or Deadshot are changed, the furor over it is going to be very small. Try changing the Trinity (or any iconic minority character in regard to mainstream audiences), however...But in cases where the changes aren't organically arrived at, it doesn't really mirror true diversity. If we change Wally or Deadshot, that doesn't change either of them into strong examples if diversity. Comic book Wally and Deadshot are still white or at least still have decades' worth of comics portraying them as white. So, it doesn't mean DC is suddenly more "hip," it just means they dont have faith that their existing or new minority characters could appeal to the general audience.