I read a very interesting article that suggested since this is ultimately a 'Man of Steel' sequel, Affleck probably wouldn't be blamed if the movie under-performs.
I still have to wonder about this news, does Warner Bros. not trust Henry Cavill to carry a MoS sequel by himself?
I think this movie is WB's reaction to MOS not being a bigger hit. It did well enough for a sequel, but not well enough to blow everyone's socks off. The mixed reviews certainly didn't help. They are throwing Batman into MOS2 to create more hype behind it.
The movie made 800,000,000 dollars world wide. How much bigger did they want it?