Right, and I agree with you. But saying: Niche Sci-fi movie A, without any stars, is a bomb but so is Niche Sci-fi movie B, which had a big star. Isn't what would, in the eyes of a studio, convince them they should take the risk on a minority. If anything, it paints the notion that the problem isn't the casting, but the genre itself. Such is the risk-adverse nature of studio filmmaking.
The success of the recent Star Wars films, and probably most notably, of the Fast and the Furious movies are much better examples of what diversity and minority talent can do for a film's box office, rather than the fact that Edge of Tomorrow and Pacific Rim were equally dissapointing.