The actual sequel to this announcement is the return of Christ in ch19 vv11-16. Matthew ch24 v30 (RSV) describes him as the Son of man"coming on the clouds of heaven with power and great glory". Revelation has the same intent when it calls him the Word of God accompanied by the armies of heaven. "It is done" means everything has happened that needs to happen in preparation for that event. 

That includes the gathering of the kings of the world for the great battle in which God defeats them all in one go (echoing Old Testament pictures like Joel ch3 vv11-16). We do not see the battle itself in ch16. They gather for battle, but they don't fight. Rather, the return of Christ in ch19 IS the battle of Armageddon, from God's side. It is the final defeat of the forces of evil.

I can only call ch19 v11 the sequel to ch16 v17 if I account for what we see in the interval. Firstly, then, vv18-21 is the world's immediate reaction to the announcement. The central point is that Babylon is about to be destroyed; God is going to make her "drain the cup of the fury of his wrath".

Then, as happens more than once in Revelation, there is a pause for reflection between the announcement of an event and the event itself. Since Babylon is about to be destroyed, the time has come to explain what is meant by "Babylon", which occupies ch17. There follows ch18, which is occcupied by a mock lamentation for the fall of Babylon. Finally, ch19 vv1-10 is an outburst of anticipatory rejoicing. This covers the fact that Babylon has vanished; "The smoke from her goes up for ever" (v3). It then moves on to the forthcoming "marriage supper of the Lamb" (v9) when Christ is united with his people. 

The short answer to the question is that the world existing before the return of Christ is now "done", for practical purposes. Everything has happened.

As for your final question, I might add that this answer has been summarised from my own book, "Silence in heaven", subtitled as "a survey of the book of Revelation". I believe the book sheds light, but of course I'm biased.

P.S. I may as well quote from it (p110);
"The effect of the seventh trumpet was the proclamation that the end had come, reaching the last minute of the world. The effect of the seventh bowl is the proclamation that the end has come, reaching the last second."