I don’t see many lefties talk about pre-modern history outside of colonialism so I am curious as to how Marxists view it.
The fall of the Roman Empire wasn’t one singular event but a series of events spanning from what I can tell starts at Third Century Crisis to its final climax at Constinantinople in 1453.
In my opinion Rome was in some ways the first proto-capitalist society. As others have pointed out it stood apart from other civilizations that came before it in the fact that it was fanatically pro-creditor much in the same way modern capitalist societies are. Not coincidentally, this society was only able to be built on genocide and imperialist expansionism.
Once that expansion stopped, the internal contradictions which come with such a socio-economic system started to accumulate: increasing wealth transfer from the bottom to the top, consolidation of land ownership into a few big latifundia, the dispossession and enserfdom of the small farmer or their migration into the cities as an impoverished urban proletariat, etc.
These contradictions built up until inevitably the entire rotten structure collapsed into repeated internal crises exacerbated by outside factors like climate change, epidemics and migrations (the latter two very likely linked to the former), though these external factors would not have been sufficient to take it down had it not already been in a terminally weakened state.
As for when the Roman Empire ended, i am in the camp that strongly opposes the view that the medieval Byzantine empire was a continuation of Rome. It was too socially and culturally different from the Rome of the classical period. Therefore i don’t agree with putting the date at 1453 though i think it is debatable exactly what the date should be.
For me there are three notable milestones that each could be counted as the “end of the Roman Empire”, these are: 395 when the empire permanently split on the death of Theodosius, 476 when western Rome fell, and 565 when the last real Roman emperor Justinian died, shortly after which the Byzantines lost control of most of Italy.
If you want to really stretch things you could maybe go all the way up to the mid to late 600s with the switch to the theme system, though that in itself was an administrative response to social, demographic and economic changes that had already occurred. Either way it’s safe to say the Roman empire proper ended sometime between the early fifth and mid to late seventh century.