[Titus Quinctius Cincinnatus]

Good analysis, but I would quibble with one small point - “The East certainly had more defensible borders with the Danube and the Hellespont”

The Danube was no barrier. The Goths crossed it with ease, and the Bulgars used it to hit the Byzantines time and time again.

[Antoninus]

The Danube was no barrier. The Goths crossed it with ease, and the Bulgars used it to hit the Byzantines time and time again.

And really, they didn't have more defensible borders at all. What they did have was enough riches to buy mercenary armies to defend them, and to bribe hostile barbarian hordes into heading West. And later, they had a heartland (Thrace, Isauria, and Anatolia) where hearty warriors could still be recruited internally in large numbers.

The West had none of these luxuries. The western heartland (Italy, Africa, Gaul) had been steadily denuded of its warrior class during the crises of the 3rd and 4th centuries. The massive western armies that went east under Constantine and Julian in the 4th century could no longer be raised by the 5th.