‘What were the main reasons for the decline and fall of the Roman Empire by the 5th century CE?’

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‘What were the main reasons for the decline and fall of the Roman Empire by the 5th century CE?’
This is a straightforward discussion question for which there are no simple answers. Historians have pondered this question for centuries and modern historians have still not been able to reach any overall, agreed explanations for the fall of what was one of the greatest empires the world had seen.
There are numerous theories and explanations- but almost all have weaknesses at some point in their assessment of the facts.
One famous account was by the 18th C ‘historian’ Edward Gibbon who spent decades studying every detail of the history of the empire and published a massive, multi-volume survey The Decline and fall of the Roman Empire. [It sits on the shelves of most university libraries, but few students ever get to read it in its entirety.] Most get to know of one of the central themes that Gibbon presents as a cause for the collapse of the empire. Gibbon argues that the empire succumbed due to “the triumph of barbarism and Christianity.”
Gibbon was referring to the onslaught of the Germanic tribes and Huns that penetrated the Roman frontiers and eroded Roman power and culture. Gibbon also believed that the values and doctrines of Christianity that spread in the empire until Christianity was eventually recognized as the official religion of the empire, those values also eroded Roman martial values and fighting spirit. In effect, it weakened Roman’s spirit to resist and emphasized other-worldly concerns (the after-life.)
Other experts have argued that the Germanic tribes did not destroy the empire. That, for the most part, they came to try and preserve Roman civilization; to share in its riches. The ‘barbarians came to enjoy, not to destroy the empire.’ Is this true or accurate?
Yet others have taken the position that the Roman Empire never really fell at all, it merely evolved in new directions after the 5th C; continuing to preserve its culture and traditions in ‘successor states’ like the Byzantine Empire as it becomes known. [The surviving Eastern Roman Empire as it was initially known.]
So this sets the scene for you to begin assessing this complex problem. The above is merely a brief sampling of a much wider array of assessments that have been made of this problem. As part of your response, you might consider why the problem is still such a ‘hot topic’ of investigation to this day? What is its relevance to us?
Chapter 6 and 8
McKay, Hill, Buckler & Beery, A History of World Societies. (9th Ed.)