 A region can be completed. You can think in terms of closure and completion, if that's the definition of your ministry. The gospel can be, as he said, fulfilled, even though lots of people are left to be evangelized. They're not all saved, but local pastors like Timothy, being left behind at Ephesus, can get the job done with evangelism. Paul's finished. He's out of there because his job is put the gospel where it's never been heard.