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Original Description:
 
“I think only Caesar could lead the Legion. I’ve never met anyone who could take his place. I couldn’t. I never had a mind for logistics. I don’t know Lanius, but from what I heard, he has no interest in leading anyone unless it’s in battle. No. The Legion dies with Caesar. What follows now are just the last steps of a man who does not yet realize… he is walking dead.”
 
-Joshua Graham, Fallout: New Vegas
 
 
The only reason Equestria managed to last so long was because it was just three species, and they saw Celestia as a goddess not just leader. Twilight did not have the same presence, and did not take the proper precautions.
 
But maybe I’m wrong, recent info shows this place is called “Ancient Equestria”
 
So think of G4 as Ancient Egypt and G5 as Modern Egypt, in a sense. For all we know Canterlot is now like Thebes and Alexandria, grand cities in their time…reduced to rubble.