So what does happen in heaven and on earth, according to the Book of Revelation?
All sorts of crazy things. A third of the land is burned up, a third of the sea turns to blood, a third of the stars become dark. There are earthquakes, and stars that hit the earth, and smoke and locusts and plagues, and hailstorms, and disappearing mountains, and a good deal more blood.
What's up with all that?
Fr. Corbett's suggestion is this: That God's promises are so wonderful the earth as it presently exists cannot contain them.
If God's promises amounted to nothing more than literally giving a certain stretch of land to the descendants of Abraham forever, to literally granting His faithful ones long lives, prosperity, and lots of children, then the earth as we know it can pretty well suffice. But His promises, as given in their fullness by Jesus, are so outrageous, so over the top, that the very world we inhabit needs to be remade for them to be fulfilled.
The language of Revelation is the language of the very world we inhabit coming apart, from below and from above and from the sides.
The new heaven is also required: what happens on earth happens in heaven, and who wants a heaven where a third of the stars are missing?
From this perspective, Revelation describes not just a battle where the field belongs to God's victors, but one where the very battlefield joins in the fight, passes away, and is recreated anew. The Apocalypse, expressed apocalyptically, isn't the sober separation of sheep and goats or of wheat and tares, it's a full-scale cosmic blowout.
And not (if we follow Fr. Corbett) due to how closely matched the forced of goodness and evil are. This isn't Ragnarok or Hamlet, where everybody destroys everybody else. This is the fulfillment of God's will; it's a blowout because the cosmos God intends to endure forever is far more wonderful than the cosmos that exists now.