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.