That was my initial feeling too, though I was always on board for Batfleck, back as far as Daredevil. That movie sucked, but he wasn’t the problem with that film. He could work as Matt Murdock. So the next logical leap, if he can play Daredevil, could he be Batman? The answer yes.
Then, once you hear the direction Snyder was going, you understood why an older Batman. Robert Pattison may be good. I’m looking forward to seeing, but he wouldn’t have been right for The Batman of Snyder’s-verse. He’s too young.
And, my well document issues with Snyder aside, I give the guy mad props, because his Batman, is the only Batman, that looks like Bats. The way I’ve always wanted to see him. A costume that looks more functional and not all plastic armor. It’s trying to seem like fabric of some sort.
The fact that he’s swinging all the time. I love that! No other film has done that, like the comics, it’s how Batman flies through the city. Web-slinging before it was a thing. And BvS has issues too, but that Batman scene saving Martha, is Amazing! It’s Batman as I live and Breath. One last thing, thank you Snyder for giving Bats a voice modulator, so actors don’t have to do cheesy Bat-voices -CB. All these things should be in Batman films from now on.
Bringing it around, Affleck was the perfect embodiment of this version of Batman. He nailed the physicality of Batman and the subtlety of Bruce Wayne, and shows growth as a middle aged Batman. I wish we could see the Justice League on screen together again.