I suppose the Middle Earth in Frodo's times are a reflection of early medieval times. And to some extent that's how it was. Collapse of Western Roman empire was a time of terrible economic downturn.
The fall of the West was changing the name on the door. Theodoric could have been a western emperor for all intents and purposes. Even the institutions remained pretty much unchanged.
The only real distress was that the loss of Northern Africa as the breadbasket for Rome meant that Rome itself continued to shrink/decline (a process which had been underway for a long time by the end), and that was only because of the lack of a grain dole, which had been in place for 500ish years anyway, because even the late Republic had significant problems with wealth inequality.
If you want to talk about economic downturns, currency devaluation after mining all of the silver in Iberia, the crisis of third century, independent administration of the East and concentration of the wealth of Egypt/Syria into Constantinople, and the amount of wealth which went into keeping up auxillia/mercenaries are better targets, and those all long predate the fall of the west.
To Briton the Western Roman empire fell hard. There was a marked collapse of urban life, a shut down to bulk trade networks which were relied on for a great amount of the economic activity. Students were no longer sent to "Rome" (or the other metropolitan areas) for education. And worst of all the Roman Military left.
This led to a complete collapse of the system, people went back to sustenance living with the drop in living quality that this, suffered raids from the Picts, Irish and Germanic people's. There was a huge power vacuum that led to local 'warlords' rising in power and a politically fractured island. This also led to the Anglo-Saxon invasion.
It is from the perspective of (comparatively) modern scholars that we get the story of the "Collapse of the Roman Empire" because from the perspective of the British isles is was a collapse.
The focus on the Crossing of the Rhein and the Romano-Briton contenders for the throne seemed a little too conveniently dramatic. Having said that, I haven't read any more scholarly books on this period of history that contradict Baxter’s narrative and reading Coalescent book was a great way to learn about the various factors that caused the collapse of the Empire on the western fringes of Europe.
https://fallofcivilizationspodcast.com/2019/01/21/episode-on...
Here is another on a general fall of Rome: https://open.spotify.com/show/4uxl02KkkbvH8kYxWtKs8b
Warning its got about 20 hours of content
The Anglo-Saxon Chronicle (dubious as it may be sometimes), Gildas, and archeological evidence from Essex really don't agree with "fell hard". It very much appears that the empire didn't have enough resources to continue propping up a fringe border province from the death of Julian the Apostate onwards, and that Brittania brought in Anglo-Saxon foederati to help out, who filled a power vacuum after Constantine III left (and possibly invited others to settle).
Particularly under Honorious, fighting between the Vandals and the Franks tied up almost all of the Roman resources in Gaul, which mattered in a way Brittania did not, and the population started to move into fortified settlements, especially after Constantine III took most of the legionary troops off in rebellion. This isn't really that different from what was happening in Gaul.
Fifty years before the "fall" of the West, there was no meaningful military presence in Britain which was not foederati, and before that, the situation was so precarious that troops revolted, leading to its abandonment. As early as the crisis of the third century, Roman Britain's economy shifted to become far more regional, since long supply chains through Gaul were untenable.
The primary exports were extracted minerals. It was, at best, colonial exploitation in antiquity. Raids from the Caledonians had never stopped, ever. Britons convincingly repelled an Anglo-Saxon invasion in 490, and it was primarily peaceful after that (and before that). Balkanized, yes, but hardly subsistence living with roaming bands of invaders.
This isn't any different than northern Gaul, or southern Gaul as the Burgundians, Visigoths, Franks, and Vandals cycled through. Or Illyricum. It happened earlier, but not like you describe.
It's incredibly telling that Gildas had both the time and education to write a somewhat polemical history less than a century after a "complete collapse of the system, (where) people went back to sustenance living", in classical rhetorical style, wherein he mostly seems to regard Britons as still being Roman (despite the "fall" of the West by that time, and the abandonment of the province 100 years earlier), as a Christian, which also tells us that Christianity had not been abandoned yet.
This is utterly at odds with your views. Where and from whom did Gildas receive his education? Why should we disregard his accounts of Mons Badonicus? Why should we disregard the Anglo-Saxon Chronicle and archaelogical evidence from Essex, Wessex, and Kent?
This narrative is false.
I always think of history as kind of a tapestry, and filling in pieces (neighboring states, for example) can be reasonably informative.
If I were working backwards, since you're into podcasts, The History of Rome is obviously quite good, and The History of England essentially picks up the historical narrative with the Anglo-Saxon states prior to the Norman invasion. Having a good, digestible background (through these) makes the "missing middle" easier to fit in. There are very few easy/readable histories of it, but there are good histories of the Merovingians, for example, which can inform a lot about the neighboring region.
It's more a question of the elves spontaneously losing the will to live.
Elves do not physically die, but they will enter comas and their souls will appear in the house of one of the Valar. The Silmarillion records this happening to one elf whose comatose body is maintained (but unresponsive) while their soul can be visited and conversed with.
[1] This fails to explain how the Mirkwood elves can live there. But the answer appears to be that they can't, since they leave along with the other elves.