In the English language, the word then suggests that what is described in the present paragraph happens after the events of the previous paragraph. So the reader is (subconsciously) prompted to cast the events of Revelation 20:1ff into the future as if its details describe things that are to happen in the time between today and Christ’s return.

It is true that the word then (the Greek has simply the word and) underscores that John saw the vision of Revelation 20:1–10 after the vision of the white horse in Revelation 19:11–21. But the fact that John saw the vision of Satan’s arrest after the vision of the white horse does not mean that the arrest happened chronologically after the triumph of the horse (and hence after the birds had their fill of the dead). In fact, as we will see, Satan’s arrest happened with Christ’s victory on the cross. The events of this paragraph span the entire New Testament dispensation.