I would wager that Henry Cavil could have saved it from being a bomb. He seems to be the only draw that DC has been able to rely on, and he’s popular with a lot of fans.
Even if you just had him as the muscle, people would have complained that his role wasn’t big enough but they would at least have gone to see it.
*Writers room. Dimly lit. Smoke hangs out from endless cigarettes butts. Their embers burn through the shadows. Their is a heaviness in the room. One writer finally speaks.*
Writer 1: "Keaton decides to don the cowl one more time, for his biggest adventure ever. To rescue Cavil's Superman and help him save the world from Zod in an epic finale."
He'll even say this awesome line: "I'll help you get this "Superman". Then, you're on your own!"
*They all look at one another. The heaviness empties from the room. A light begins to break through the darkness. They all smile. Another writers speaks up.*
Writer 2: Ok. So no to Superman. No to Keaton saving the world. They both die.
*Everyone looks around. They smile again. Heads nod in agreement*