What Bird Box fails to accomplish is what "Annihilation" (Starring Natalie Portman, 2018) does.
While the concept and visuals are compelling and gut wrenching, how the story pans out is just not that.
The two time frames shown in the film are useful to let viewers make predictions and keep interest in what's happening, but that's really all that keeps us watching. Unlike "Annihilation", Bird Box's "grand" finale is a letdown to the tense horror that is built up in viewers minds from watching the majority of the film. There is no final showdown between Malorie (Bullock) and the unseen entities, as the last few minutes is devoted to the finding of a sanctuary, similar to places seen in AMCs The Walking Dead and other zombie/evil forces shows.
In "Annihilation", Portmans character faces off with the inhuman creature that caused a disturbance in her world, and we are also left with a cliffhanger ending. Bird Box has a cliffhanger too, but it's a cliff that's 2 meters from the ground. Malorie is safe in the sanctuary, and that's about it.