Actually, i would be a hell of a lot more optimistic if that happened.
All he needs to do now is take over the directing.
Man of Steel should have basically been Batman Begins (with Supes obviously) - a character driven story, detailing one hero's journey of emerging from the shadows to being a shining light for his city's inhabitants.
However, what we got was a complete CGI-fest of absolute chaos. True, Snyder cant take all the blame, it was very poorly written too, but ultimately when things go wrong, it all comes back on the director.
Personally, i think Affleck will be fine as Batman. Hes had a career of ups and downs (as an actor) and his acting ranges from terrible (Pearl Harbour, Gigli) to bordering on excellent (Hollywoodland, The Town). For the last seven or eight years now (pretty much since he started directing himself) he has been consistently solid as an actor.
For me, Snyder and Goyer are the problem.
"Don't you do it, Hitler.....don't you dare fall in love with me"