The two best Horror movies to come out last decade 28 Days Later and Descent both had to alter their bleak endings for the American market. [REC] and its inferior remake stuck with the original bleak ending. As early as the 70's Romero struggled with this, Night of the Living Dead had a shockingly bleak ending, but Dawn's ending was changed in the last moment from everyone committing suicide, to a more ambiguous escape into sunset.
I think a bleak ending is OK as long as it is earned, you earn it with good characterization and acting, so when the heroes are defeated, you see it as tragedy as opposed to more gore.