It seriously baffles me how anybody could possibly think a movie with Batman and Superman together on screen could possibly fail, people have been wanting to see that for decades. Add on to that Wonder Woman, Aquaman, Cyborg, Flash, and the fact that it's a set up for the JL movie, and you've got even more stuff people want to see on the big screen for the first time.
Hell, the Guardians of the Galaxy and Deadpool were both huge hits, and they don't have anywhere near the pop culture presence as Superman, Batman, Wonder Woman, Aquaman or The Flash. Cyborg might be closer to the GotG and Deadpool's level, but I think the other characters can cancel out his lower pop culture presence several times over.