Think about a city having enough soldiers to control the land and seas they controlled. Then they spread and made more citizens. More soldiers. They had to give them land in most cases especially after significant victories.
By the 4th century they were paying off mercenaries to control the outer regions especially in north and eastern europe. That money couldn’t hold forever. Also more population in Germany and other places. The fall was inevitable.
One way t understand the fall is to look at europe today. If 5 countries with armies aligned against Italy could they control all of europe?