I still maintain that Affleck is the best cinematic Batman we've ever had, but I think WB made a mistake going with the older Frank Miller version. If this is supposed to be a shared universe with the same kind of longevity as the MCU, they'd better make sure the actors are committed.
RDJ was kind of a fluke for Marvel since his career was in the toilet before he made Iron Man while Affleck's was already in an upswing after The Town, Argo and Gone Girl.