Eternity says that humanity, inevitably, will destroy Earth, so he does it. Then he immediately replaces it with an exact duplicate.
This was the first time Marvel rebooted its universe, even though it didn’t change anything at all except for Dr. Strange nobody is their original self.
Plutarch and Plato debated the Boat Problem: If you replace every plank and board on a ship, individually, is it still the same ship? I think this is what Englehart was trying to do here. Only it doesn’t work real well.
At the end, The Ancient One goes away to become part of everything-sort of like becoming Dr. Strange’s Obi Wan.
Yes, in the end Strange has to “come down.” Because clearly this issue was just an Englehart acid trip.
Note: In issue #19, it is revealed that this entire story is bullshit.