I totally understand his decision. I read an interview where he was talking about the logistics of directing a movie while wearing a 60 pound rubber suit, and how impossible it would be to do his job for 14 hours a day acting and doing behind the scenes stuff.
Forbes had a list of directors apparently being looked at to direct in Affleck's place. George Miller, Matt Reeves, Denis Villeneuve and Gavin O'Connor are all listed as possibilities. I'm personally hoping for O'Connor, as I loved Warrior and thought the Accountant was really quite good. Gerorge Miller would be interesting, but I'd rather he keep doing Mad Max stuff, rather than dipping his feet into another major franchise.