In short, Rome was a city. The question isn’t why did it fall, the question is how did the empire stay together so long.
You can’t hire your enemies as paid mercenaries and think you are secure.
The hinterlands population increased greatly from 500 BC to 400 AD Demographics is a MoFo.
Splitting the empire into thirds is a non-starter.
My 50 cents worth.
No, but you CAN award your troops prime farmland in the areas you conquer, where they can raise a large family whose sons will want to serve a stint in the Army so that THEY can get land of their own.