I believe the Roman Empire is considered to have fallen when Theoderic the Great became the ruler — he was a Goth, not a Roman. It was over at that point.
Yes, great civilizations have a short life of a few hundred years, and then they rot from inside. We had a good run. I say we should colonize Antarctica— we are strong and can survive the cold. We could turn it into a wonderful place to live. Just filter who you allow in. No muslim filth, for starters. No low-wage immigrants