Honestly, I think Affleck can do the role just fine. He's not the same guy he was when he made Reindeer Games and whatnot. He's matured as an actor and as a human being, it seems, and I think that'll show in his performance.
Anson Mount, the guy who plays Cullen Bohannon on Hell on Wheels, had an interesting thing to say about his portrayal of that role. He basically said that he didn't think he could've played it when he was younger. Even if he'd looked the part, he'd lack the life experience necessary to give the role the weight it needs. I think Affleck has that experience now and can give the role weight.
Look at it this way. Was Bale's portrayal of Batman/Bruce Wayne all THAT amazing....or was he borne up by the script, direction, and how the production team really took the films seriously instead of making them campy? I think Bale was fine, but he wasn't AHMAZING. The FILMS were good, though, and Bale didn't screw that up (well, except for the voice on occasion). With Affleck, IF they've got a good writer and director, I can see it going just fine. On the other hand, with a lousy writer and/or director, it won't really matter who they cast, will it?