I doubt that casting Saldana in any movie at all would hurt its box office performance. Who do you think is really going to pass on buying a ticket to Superman because Lois Lane isn't a brunette of european background, other than the white racist yahoos who tried to organize that tiny little protest over "Heimdall" in Thor?
True fact: racist or not, a black actress in a movie headline with a white suprehero ain't gonna bring much money from overseas or
in the good ol' USA. I also find it funny that a non-white literary character can be changed to white and the uproar can deafen you, but a black actress for a white literary character is absolutely fine. Strange, that!
Every fanboy who's so incredibly narrow and anal as to object to the casting on "continuity" rather than explicitly racist concerns would be offset by hundreds of folks who'd see the ads and go "oh, I know who she is and like her. Let's see that." This is, of course, the only reason that well-known actors command high salaries to begin with.
You do realize both of Zoe Saldana's movies after Avatar (Losers, Takers) were insta-flops, yes? And in Avatar she wasn't even in her true form? You can call Tom Cruise, Ben Stiller and Robert Di Niro as well-known actors. Zoe saldana ain't that.