The reason to consider trading Ender is that his value comes almost entirely from his defensive skills and he has now reached the age at which defensive skills on average tend to start to show pretty significant decline.
And of course, he's also reached the point at which offensive skills often to start to decline.
Maybe he ages gracefully, but he's probably not going to be a plus-plus center fielder when the Braves guarantee expires.
Once he's not a plus-plus defender, he's a fourth OF or possibly worse. The Braves aren't the kind of team that needs to be paying its backup OFs 8m dollars, based on recent payrolls anyway.
While the Braves do not have any immediate options for replacing him internally in the OF, I nonetheless think that Ender should be very available in trade discussions if someone has an interest. If the Braves fall out of contention, he's honestly one of the first guys they should consider moving.