First there are a couple of ideas that are thrown around above, that have largely been rejected by modern scholarship.
Christianity: Gibbon is the most famous proponent of this argument (so its an old argument). There are a couple of key problems with this argument. First the timing does not really work. The Empire largely recovered from the crisis of the 3rd century at the same time it rapidly Christianized. Further, the Eastern Empire continued for another thousand years.
Debasement of Coinage: This is a superficially plausible theory but here the timing really does not work. The debasement occurred in the middle of the third century. The coinage was restructured in the time of Diocletian and the Empire survived for another 2 centuries or so. Further there is little evidence that trade declined during this time. In the same way Diocleatian's edict on maximum prices does not work in terms of explanation.
Moreover, most evidence suggests it was not really followed.
A few things:
- In The Fall of Rome and the End of Civilisation, historian Bryan Ward Perkins presents evidence that trade activity did indeed decline markedly in the Late Empire compared to the period 0 to c.200AD. The evidence from shipwrecks and coin finds IIRC shows a significant reduction. It wasn't limited to the third century.
- You are right about Diocletian's edict, that it was too early on its own to account for Rome's fall, and that it wasn't followed/could not be enforced. However, Diocletian's edict was a symptom of an ongoing problem, namely inflation and debasement. Looking into this a little more, it seems Rome didn't have sufficient sources of silver, although the reasons why the coinage was continually debased are not fully understood. One possible explanation is an imbalance of trade. As time progressed, the trade deficit of the west, because of its buying of grain and other commodities, led to a currency drainage in Rome. It's certainly a topic that merits further research, but I don't believe one can dismiss it as a factor. Unless drastic evidence has come to light since I last looked at this, around 2006.
- Christianity is probably the weakest explanation for the fall of Rome of them all. The simple fact of the survival of the Eastern Roman Empire easily disproves this one. That said, the role of Christianity in changing the character of the empire may not be totally irrelevant. For instance, Christianity certainly didn't help the Byzantines in the early 7th century, when Byzantine religious persecution of Monophysite Christians and Jews played a role in the permanent loss of the entire east and south, including Egypt, Syria, Palestine and North Africa.
Did the Western Empire Fall: This remains a popular idea amongst some academic historians. The basic idea is that the Western Empire did not really fall; rather it transformed into something different (medieval civilization or something). I will admit that I do not have deep expertise in this strain of thought but its strikes me as profoundly implausible... The sense I get is that this idea is declining in popularity in academic circles.
Oh god I hope so. This one was one of the theories that I found ridiculous when I was a student. That silly Peter Brown and his theories of 'gradual transformation and change'. What a load of nonsense! I hope you are right. It would please me greatly to see this revisionist garbage consigned to the dustbin of history. I am totally in the Bryan Ward Perkins "The barbarians invaded, it was bloody and brutal" camp. It wasn't all a process of peaceful accommodation and change (though obviously that sounds nice). The reality was surely much worse (sadly).