How did the Roman Empire decline for the last 4 centuries of its existence?

So we all know about the fall of the Roman Empire in 476, but I think the real question here is how it managed to persist for a few centuries beforehand. <br><br>The Rhine and Danube frontiers were let through multiple times beforehand. In 378 half of the empire's legions were wiped out in one battle and still the empire persisted. <br><br>The Sack of Rome in 410 was a psychological blow, and certainly the Vandals sacking of Carthage in 439 and then Rome in 455 were also major setbacks.<br><br>&#x200B;<br><br>Then we have the empire splitting in 395, and West losing a significant amount of territory for East, and then West proceeding to just completely squander all of that, and still going on for another 80 years. <br><br>So how did it manage to keep on going for so long?