I would like to slightly argue with this proposition regarding the fall of Rome.
It is indeed true that the direct reason for the fall was the weakness of the late Roman armies compared to barbarian forces. But Rome moved away from using farmer soldiers as the backbone of the army with the Marian reforms in 107 BC. This did not stop the expansion of the empire nor weakened the army for several centuries. Q.E.D.
However, I think your speculation in the second part (transition of power) is actually a really good explanation for this decline of the Roman army. The Imperial armies often rebelled in the late period, trying to promote a new Emperor. To counter this, reforms were introduced that decreased the chance of a successful army rebellion, but they also greatly diminished their effectiveness: ”Under the Roman emperors, besides, the standing armies of Rome, those particularly which guarded the German and Pannonian frontiers, became dangerous to their masters, against whom they used frequently to set up their own generals. In order to render them less formidable, according to some authors, Dioclesian, according to others, Constantine, first withdrew them from the frontier, where they had always before been encamped in great bodies, generally of two or three legions each, and dispersed them in small bodies through the different provincial towns, from whence they were scarce ever removed but when it became necessary to repel an invasion. Small bodies of soldiers quartered, in trading and manufacturing towns, and seldom removed from those quarters, became themselves tradesmen, artificers, and manufacturers. The civil came to predominate over the military character, and the standing armies of Rome gradually degenerated into a corrupt, neglected, and undisciplined militia..” Source: https://www.marxists.org/reference/archive/smith-adam/works/wealth-of-nations/book05/ch01.htm (The link is to the first chapter of Book 5 of the famous Wealth of Nations by Adam Smith. I suggest reading it, it has really good insights on the nature of armies.)
I would like to slightly argue with this proposition regarding the fall of Rome.
It is indeed true that the direct reason for the fall was the weakness of the late Roman armies compared to barbarian forces.
But Rome moved away from using farmer soldiers as the backbone of the army with the Marian reforms in 107 BC. This did not stop the expansion of the empire nor weakened the army for several centuries. Q.E.D.
However, I think your speculation in the second part (transition of power) is actually a really good explanation for this decline of the Roman army. The Imperial armies often rebelled in the late period, trying to promote a new Emperor. To counter this, reforms were introduced that decreased the chance of a successful army rebellion, but they also greatly diminished their effectiveness:
”Under the Roman emperors, besides, the standing armies of Rome, those particularly which guarded the German and Pannonian frontiers, became dangerous to their masters, against whom they used frequently to set up their own generals. In order to render them less formidable, according to some authors, Dioclesian, according to others, Constantine, first withdrew them from the frontier, where they had always before been encamped in great bodies, generally of two or three legions each, and dispersed them in small bodies through the different provincial towns, from whence they were scarce ever removed but when it became necessary to repel an invasion. Small bodies of soldiers quartered, in trading and manufacturing towns, and seldom removed from those quarters, became themselves tradesmen, artificers, and manufacturers. The civil came to predominate over the military character, and the standing armies of Rome gradually degenerated into a corrupt, neglected, and undisciplined militia..”
Source: https://www.marxists.org/reference/archive/smith-adam/works/wealth-of-nations/book05/ch01.htm
(The link is to the first chapter of Book 5 of the famous Wealth of Nations by Adam Smith. I suggest reading it, it has really good insights on the nature of armies.)