Blaming it on the barbarians is a classic argument, and it's not
entirely out of favor yet. The barbarians
were given citizenship, technically, in the 212 Edict of Caracalla. It's uncertain but unlikely that this actually resulted in any changes to the way people of Germanic were
treated, but it did make those people eligible for taxation and draft. If I were to blame Germanic citizens of the empire for anything, I would first sympathize with them for being, like black folks in the early 1900s U.S.A., technically free and equal but really "separate but equal".
Not sure what paragraph to stick this bit in, but some "barbarians" weren't even Germanic, they were simply unpopular Romans or descendants of exiled Romans who wanted "their" family holdings back.
The part that's a little clearer IMO is that the historian from which most "it was the barbarians!" arguments are descended, Edward Gibbons, had a huge bias when writing
Decline and Fall of the Roman Empire. He was more impartial than other historians of the time, which is great, however his disdain for the commoner comes through strongly throughout the text of his work. This makes him a normal person for the 1700s in which he lived - even our over-worshiped Founding Fathers were generally tolerant of slavery, and their desire to extend rights and citizenship to "all mankind" only fully encompassed white males who owned land. He believed in Empire and he specifically believed in the British Empire, so in describing the slow collapse of Western Rome he was quick to blame it on externalities and devoted no ink to addressing whether problems endemic to imperialism and to
primitive accumulation of capital by the elites could have been factors. His narrative was written in, and for, the British Empire and has to be analyzed as such in its context of pro-imperialist, late-Enlightenment times.
It's a
fossil at this point and with very few exceptions not taken very seriously at this point except as a pioneer of the modern practice of focusing on primary sources. He did a great job tracking those down and using them to support his worldview.
Anyhoo, there are at least
210 reasons people have come up with for the fall of the Roman Empire. Some are easily dismissed, like ascetism or homosexuality, but more than one of these causes were probably factors. Targets and factors shift depending on whether you're focusing on 476 as the last whimper, or going back to 291 to trace the origins of the decline (and while I'm a fan of that, it overvalues hindsight and discounts the possibility that at any point between those years, a reform or restructuring could have turned things around).
So if Edward Gibbons is a relic, who's got their finger on the truth? Beats me. Ian Morris is a funny figure with
Why the West Rules -- for Now. I look forward to the day when people stop writing as if the West is "us" and the East is "them" and the day when we're not locked in some stupid schoolyard competition while our planet falls apart around us. I mention him because his approach is relevant to the question you and I are sorta playing around with: "What factors in Rome's collapse might apply to the US and more generally to Capitalism / 'the West'." More seriously,
The Fall of Rome: And the end of Civilization is great because it is willing to explore multiple possible factors of collapse:
Per those folks, drought and famine, like you say, were a big factor - hello California! - but as we see with Bill Gates or to a lesser extent Richard Charles Branson, the application of even a fraction of an elite's wealth to the problems of the poor can save and improve the lives of millions. So I'm still partial to blaming it on Roman generals, who were fond of calling themselves Emperor, sometimes multiple Emperors at the same time. If their wealth was in the hands of the people - or, at worst, in the hands of a centralized not-entirely-shitty government - I believe that it would have been easier to defend against encroaching neighbors and use the surplus wealth to careful wait out the drought. But, in my opinion, because most of the wealth was concentrated in the hands of generals/warlords, who hoarded it when they weren't at war and spent the majority of it on troops and armaments used against fellow Romans, which allowed multiple other factors which could have otherwise been dealt with to erode the foundation of the empire while the schoolyard competitions raged on.