Rome was already in terminal decline by that point. A long series of ruinous economic policies, starting with the debasing of the coinage several centuries earlier and culminating with a bunch of bizarre, socialistic edicts later pretty much guaranteed that Rome was on the way out by the time Alaric made it official.
The History Channel looked at this. Compared the Roman soldier of Caeser’s times with that of 400 years later. No comparison. Caesar’s soldiers were heavily armored brutes, trained to ruthless efficiency and armed to the hilt. 400 years later they were conscripts from outside Rome, equipped with straw armor and joke swords. Kind of like what Obama is doing to our military now.