>> All of the things you listed were present in their society but I think that you left off the principle reason for their demise. Their ruling class used their political power to excluse themselves from paying any taxes. Eventually the poorer citizens, tradesmen, and farmers could not pay all of the taxes needed by the state. This brought on the feudal states when the middle classes gave over their land holdings to the exempt ruler for a place on their growing estates. As the central government got weaker these rich citizens formed their own little kingdoms. It kind of reminds me of the corporations that send their factories over seas to avoid paying Americans a living wage, and in order to avoid paying taxes to support our country.
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Well that sounds like another factor in weakening the Roman state.
But I think my reasons are more important.
Feudalism was important to preserving society in the Dark Ages.
Feudalism was well established in the Roman World even as early as 400 A.D. so that lends some credence to your claim.
You cannot ascribe a single factor as cause to the decline of the Roman State and Empire.