It was always going to be the case that Warner Bros. would look to bring Batman back as soon as possible. Christopher Nolan's Dark Knight Trilogy had dominated the cultural landscape for close to a decade, and while any successor was always going to have a hard time under the cowl and living up to the vision of that saga, WB didn't make life any easier for their chosen candidate.
While it's equally true that Affleck hasn't done nearly enough to assert his Bat-credentials, introducing him in an overinflated comic crossover just wasn't the way to do it. The Dark Knight Rises was barely four years old, and thrusting the Caped Crusader into the mix so early on felt like a cynical measure by a studio lacking confidence in their own burgeoning franchise.
Batman sells - of course he does - but so does the idea of Batman. Much like The Force Awakens delayed the arrival of Luke Skywalker, Snyder et al. could've quite easily saved Bruce Wayne for another day, placing him centrally to the wider narrative of the DCEU whilst affording these newer heroes space to grow and develop into fan-favourite characters.
Instead, we got Batman v Superman, which managed to aggravate fans on both sides of the Bat/Supes divide for a myriad of different reasons.