The more this comes up, the less and less it seems like news, even more so after Matt Reeves' tweets.
It's not. Even he does leave, it will barely be news, despite the fact that it will get a ton of media attention. The possibility has existed for years now. Nothing will be surprising at this point after all the drama.
That said, people really need to stop pretending that anything is set in stone, and that any one source or rumor "proves" anything. It's gotten ridiculous.
Affleck may be well received in his next turn in the role, JUSTICE LEAGUE might be a massive financial and possibly even a more critical success, and his personal life and career might be going well, and he might feel better about returning to the role in the future. Or JL might not get the reception he hopes, his career may need a different direction, and he may have some personal issues, and decide he's done.
Or he may have already decided that he'd probably rather not do THE BATMAN, and wonder of wonders, he's a human being and changes his mind.
At this point, people need to be prepared for any possibility. But beyond that...it's simply not that big a deal. At this point, Affleck as Batman has been one of the single most negative sources of media attention this universe has gotten. A new actor in the cowl could remove that stigma and allow for a fresh start. Affleck committing to do another film could also have very positive results.
He's solid in the role, but I think we need another film with him playing a more traditional version of the character to really gauge his abilities in this role. But the character of Batman is bigger than one person. The character and franchise could survive if Ben Affleck decides not to move forward in the cape and cowl.