it was only 75 years or so from the year that the united Greeks turned back Darius, to the end of the disastrous Peloponnesian wars...and the first 30 or 40 years of that period was the golden age of Athens.
So, really, the decline came very, very rapidly, just a generation or a generation and a half.
Sure, Greece limped on for a while. But who would have thought the the decline from the Age of Pericles could be so total, and so swift?
Rom 1:25-32
25 They exchanged the truth of God for a lie, and worshiped and served created things rather than the Creator — who is forever praised. Amen.26 Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. 27 In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion.28 Furthermore, since they did not think it worthwhile to retain the knowledge of God, he gave them over to a depraved mind, to do what ought not to be done. 29 They have become filled with every kind of wickedness, evil, greed and depravity. They are full of envy, murder, strife, deceit and malice. They are gossips, 30 slanderers, God-haters, insolent, arrogant and boastful; they invent ways of doing evil; they disobey their parents; 31 they are senseless, faithless, heartless, ruthless. 32 Although they know God's righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.
NIV