Most of these articles and theories are BS.
Rome had divided the empire and was using mercenaries (read former enemies) and paying them to not fight. The thing is not how did Rome fall, but how did a city control such a vast empire for centuries, while pagan and immoral.
Since christianity was the official religion almost a hundred years before the fall, it can be posited that Rome was the most moral than at any time before it fell.
Lastly, in 69 AD Rome was still strong and expanding despite the “immorality” especially of Tiberius, Caligula, and Nero. So should we say the more less moral, the stronger the empire?
You are being simplistic - out of your “libertarian” bias, I conjecture.
Rome still had a martial code and great discipline in that earlier period. This culture is losing that, as did The Roman Empire.
Nations have lifespans. By the time Christianity became dominant, the natural decay of that country had set in.
Western Civilization would not have existed, and would not have produced the USA, without Biblical Christianity.
P.S.
It is A.D. (Anno Domini) 69, not 69 A.D.: In the Year of the/Our Lord 69, not 69 in the Year of the/Our Lord. (In A.D. 1969, Zager & Evans did not sing “2525 in the Year” in their hit song.)