Bullshit. The Ptolemies went to pot in Alexandria, but even so Cleopatra went down fighting. The (down-home) Macedonians had a lot of money, but they went down fighting. The Gauls lost a series of pitched battles before they were conquered, as did the Britons.
Tacitus was a remarkable writer, but he had an axe to grind. The degeneration through luxury he had in mind was back in Rome.
The Big Think? Here I want to quote G.K. Chesterton's friend, who heard what someone prefaced with a "A thought:", and said, "Good God, man, you call that a thought?"