Since it sounds like Affleck wants out. When they cast a new Live action Batman I suggest they don’t cast a Batman. Cast a good Bruce Wayne then CGI the Batman. Bonus: they could use Kevin Conroy’s voice for when he is in the suit.
Spider-Man was basically cast this way and it has worked out, more or less. Bruce Wayne is the more challenging part to play anyway. With most incarnations of the character, Bruce Wayne is the mask. When he’s fighting crime, he’s his true self. I think casting with this in mind would make for a better movie.
If Affleck does leave, I guess what happens depends on the future of the DCEU. If they are going to go to a format of single stand alone movies that are not interconnected, then your idea would work.
If WB wants to continue the EU concept then they can’t recast Batman. Give him some sort of heroic death and bring in Nightwing. Nightwing is a Batman analog with a much sunnier disposition.
You are following Robot Arm’s First Law of Superhero Movies. Any Hollywood actor can wear a spandex suit. You need someone who can be convincing as the alter-ego. Christopher Reeve was great as Clark Kent. Does anyone buy Jessica Alba as an astronaut?