I wonder if the decay mentioned couldn't have come from other causes than "decline in morality", with the latter following from the former.
For example, I wonder if Greece went through a land consolidation that disowned the small landower and the shepard of the commons. I thought I read that Rome experienced that - presumably after Polybius' time.
A new slave class, formerly self sufficient, then adopts slavish habits of "escape"...
You are correct that Rome experienced a disownment of the small landowner and it occurred precisely during Polybius' lifetime between the Second and Third Punic Wars.
I would highly recommend "The Punic Wars" by Brian Caven for a detailed history of that period. During the First and Second Punic Wars, Rome relied on her citizen soldiers. They farmed their small farms during peace and then served in the Legions when duty called to defend the Republic.
The dogged determination that Rome showed until victory during the First and Second Punic War, despite massive naval losses in the First and devastating defeats by Hannibal in the Second, leaves you in awe of the Romans of that time period. It was their "Finest Hour" and the character of the Roman people during that time reminds you of the British and the American character during World War II.
However, the Roman government failed it's citizen soldiers in the end. Although some Conservatives glorify pure capitalism, what happened to Roman society after the Punic Wars shows that sometimes government does need to step in and do what is right, not only for the benefit of it's citizens but for it's own welfare.
Rome's failing was the absence of a social safety net for the families of their citizen soldiers. When a soldier lost his life or was badly wounded in battle and the wife could not maintain the farm in economic health, large landowners came in and bought those debt-ridden properties. The children of the veterans would then grow up landless and migrate to the city where the wealth from war booty allowed the government to feed Roman citizens that were less than productive.
The results were predictable and turned a noble citizen-warrior population into,.... well......Democrats.
The final nail in the coffin of the Roman Republic came when the Legions were manned by professional soldiers who were more loyal to their commanders than they were to the Republic. Where as earlier Romans fought to defend the Roman Republic and their homes, the new breed had nothing at home to fight for so they fought for their own enrichment.
Caesar just happened to be the commander that killed the Republic. With such an Army, it was only a matter of time before some commander did so.
A Julius Caesar would not have arisen in the citizen-soldier Roman army of the Punic Wars.
In regards to Greece, land ownership by large landowners was always a problem. In Athens, Solon (the Lawgiver) put a limit on the amount of land an individual could own in order to address the disownment of the small farmer. However, Greece only had about 20% of it's land suitable for farming so the Greeks had other venues such as maritime trade support it's citizens.
What truly brought about the end to ancient Greece was it's total lack of political unity. While the Romans had a gift for uniting, the Greeks were a herd of cats. Only massive threats such as Xerxes' invasion could get the Greeks to stop fighting each other and fight for Greece as a whole.
By the time of the Second Punic War, it became evident that whether the winner was Carthage or Rome, the divided Greeks would not be able to prevent a military takeover by the winner.
Polybius recounted a speech made by one Greek at a conference convened to attempt to resolve this problem of Greek disunity in the face of the military danger that was looming to the West:
Histories, Book V, Chapter 104.
I shall report the speech that Agesilaus of Naupactus made before the king and the allies at the first conference. It was as follows:
"It would be best of all if the Greeks never made war on each other, but regarded it as the highest favour in the gift of the gods could they speak ever with one heart and voice, and marching arm in arm like men fording a river, repel barbarian invaders and unite in preserving themselves and their cities. And if such a union is indeed unattainable as a whole, I would counsel you at the present moment at least to agree together and to take due precautions for your safety, in view of the vast armaments now in the field and the greatness of this war in the west.
For it is evident even to those of us who give but scanty attention to affairs of state, that whether the Carthaginians beat the Romans or the Romans the Carthaginians in this war, it is not in the least likely that the victors will be content with the sovereignty of Italy and Sicily, but they are sure to come here and extend their ambitions beyond the bounds of justice.
Therefore I implore you all to secure yourselves against this danger, and I address myself especially to King Philip. For you, Sire, the best security is, instead of exhausting the Greeks and making them an easy prey to the invader, on the contrary to take thought for them as for your own body, and to attend to the safety of every province of Greece as if it were part and parcel of your own dominions.
For if such be your policy the Greeks will bear you affection and render sure help to you in case of attack, while foreigners will be less disposed to plot against your throne, impressed as they will be by the loyalty of the Greeks to you.
If you desire a field of action, turn to the west and keep your eyes on the war in Italy, so that, wisely biding your time, you may some discovery at the proper moment compete for the sovereignty of the world. And the present times are by no means such as to exclude any hope of the kind.
But defer your differences with the Greeks and your wars here until you have repose enough for such matters, and give your whole attention now to the more urgent question, so that the power may still be yours of making war or peace with them at your pleasure.
For if once you wait for these clouds that loom in the west to settle on Greece, I very much fear lest we may all of us find these truces and wars and games at which we now play, so rudely interrupted that we shall be fain to pray to the gods to give us still the power of fighting in general with each other and making peace when we will, the power, in a word, of deciding our differences for ourselves."