Europe degenerated enormously after the fall of the Roman Empire and took something like 8 centuries to *recover* much less start significant development again. Granted, some innovations like stirrups and the horse collar and gunpowder did happen--but many of them came from other places. Stirrups came from Mongol invaders. Gunpowder came from China.
Thedas does not have an "empire" to promote what Isabel Paterson called the "Long Circuit of Energy" (highly recommend her book The God of the Machine)--long distance trade that fosters innovation and development. In fact, it has a lot more material hazards to trading. Maybe not so much plague, but certainly plenty of other disasters. The Tevinter Imperium, with its emphasis on slavery, is in fact a massive anti-innovation, anti-development force. I don't really think Orlais is a good source for a pro-innovation force, either. The nobles have too many prerogatives that are geared toward a lower level of development and that would crumble if the peasantry started getting all innovative and trade-y. Fereldan would be a better bet but their population is too small and their economy has just suffered a major disaster.
This is something of a tired trope that only has legs because it is at least half true.
Archaeologists refer to the period immediately after the end of the Roman Empire as the "Great Simplification" for most of Western Europe, especially Britain, and chiefly for the reasons you seem to highlight: trade links collapsed, shattering regional economies and fracturing the monetary economy, while large complex governments with lots of money to throw around ceased to throw that money around.
But by the same token, "technological development" being "lost" is kinda...ehhhh. It doesn't really work as either explanation or effect.
For one thing, one could reasonably point out that the Roman Empire's technology in 400 wasn't hugely different from its technology four centuries earlier. If vast powerful states and massive quantities of trade were what stimulated technological development, then shouldn't Rome have gotten, I dunno, "more" technology?
It's these sort of schematic approaches to technological development that bother me, as a historian, because for every positive example that can be found for Reason A stimulating the creation of new technologies, there are multiple negative examples of Reason A failing to do anything of the sort. And for examples of Reason B stifling technological development, there are examples of Reason B existing in societies that stimulate it as well.
Case in point: your association of slavery with a lack of technological development. That might explain imperial Rome's apparent lack of significant innovation, but how does it explain the flowering of philosophy, art, architecture, mechanical science, and suchlike things during the Hellenistic era? All of the Greek states and Hellenistic monarchies kept vast quantities of slaves. Or take the assertions by many historians of the Industrial Revolution that slave labor-supplied raw material inputs are directly correlated with increases in productivity in Great Britain in the late eighteenth and early nineteenth centuries. Slavery didn't stifle innovation in those societies; in fact, it may have helped to enable it.
For another thing, I strongly disagree with the characterization of post-Roman Europe as being a technological black hole. This is not merely a case of agricultural, navigational, and metalworking development - although all of those things happened in the centuries immediately following western Rome's political end - but of philosophy, literature, and historical thought. Scholars like Gregory of Tours, Gregory the Great, or Boethius were just as learned and literate as their late-Roman counterparts like C. Sollius Sidonius Apollinaris, Q. Aurelius Symmachus, and Ammianus Marcellinus. Early medieval Ireland - admittedly, not technically a "post-Roman" society - may have been the most literate society in the world of its time. Documentary evidence certainly doesn't disappear (except for Britain, and even then only for two centuries). Tell any medievalist that she's studying a period full of illiterate, backwards idiots, and she will cackle maniacally as she buries you under several tons of chartulary papers. And she will also point out that apart from places like Egypt, written records have a hard time surviving the millennia
anyway, due to climate conditions as much as anything else. The quality of historical writing was fine, too, especially when you consider that
classical historians weren't that good (and were kind of obsessed with referential nonsense the only point of which was to show how well-read they were); arguably, the explosion of hagiographical works in medieval Europe - which had no equivalent in classical society - indicated a
better standard than had existed previously.
It's really hard to quantify just how much daily life changed for any given individual in western Europe. The poorest farmers were hardly any worse off; by the seventh and eighth centuries, the richest Frankish and Gothic magnates equaled their Roman predecessors in wealth. Long-distance trade didn't do incredibly well, but long-distance trade is almost totally negligible for all of human history up to the nineteenth century or so compared to regional trade, so that doesn't tell us much. Henri Pirenne's study of trade collapse in the Mediterranean is now widely discredited due to his focus on luxury items that virtually nobody would be able to acquire anyway, and irrelevant due to its total lack of archaeological data. The archaeology itself is equivocal, indicating regions of increased trade, manufacturing, and prosperity (e.g. the northern Gallic-Ardennes-Rhineland region, the core of the Merovingian monarchy) and areas where things weren't so rosy (e.g. southern Britain, inland Spain).
Tax structures did not entirely disappear; all post-Roman kingdoms for which we have data (i.e. "not Britain") seem to have taxed into the seventh century. But they slowed down and eventually stopped, not because they "forgot how" to tax, but because armies had become landed instead of directly paid, eliminating the single largest budget item in the old Roman fisc. Without paid troops, tax was less and less relevant. And this was a change that to a significant extent had already started under Rome, with the provision of the
annona and the establishment of large, mostly stationary regional armies based on specific territories. Post-Roman rulers simply cut out more of the middlemen. Is that indicative of technological decline?
You claim that some technological innovations didn't come from within Europe, but from outside it. I...am not sure that that matters. It's equivalent to a claim that modern France is a technologically backward society because the French didn't come up with the iPhone. And that's without getting into whether the stirrup and gunpowder were particularly "important" technologies - very debatable, especially in the case of the stirrup - and whether the lag time between their development outside Europe and their development inside Europe actually meant anything.
The point is that the picture of the Roman Empire, technology, and the so-called "Dark Ages" is a great deal more equivocal than you're getting across.
Thirdly, the "800 years" thing is just silly. Leave aside the matters of the Carolingian "renaissance", the creation of settled, economically complex societies across Central and Eastern Europe that didn't even exist when Rome was around, the existence of the Byzantine and Umayyad empires, and suchlike things. I'm not even sure what you're ostensibly measuring. If it's
this ahistorical farce, then, well...no. Just no.
And finally, more minor quibbles. Stirrups are first attested in European records by the Byzantine military text
Strategikon, popularly attributed to the Emperor Maurikios, who described them as being used by the
Avars, not the Mongols. That's a good six centuries off. Plague is usually described as a consequence of trade (or rather, "large-scale movements of peoples over long distances"), not an inhibitor of it. And "noble prerogatives" inhibiting peasant-based technological innovation are a total red herring; I'm not sure what peasants have to do with technological change, nor am I sure why aristocrats aren't associated with it. It's not like people lose 50 IQ points when they get titles.
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Fundamentally, though, comparing Thedosian technological change to historical technological change is pointless. The setting has seen relatively little technological change not because of historical processes, but because of the demands of the story. Fantasy settings usually have an ahistorical approach to technology because frankly it's easier that way. If things don't change much, the timeline is clay: it can be manipulated at will without regard to factors like new technology. The setting can still be described in the same way, rendering it easily recognizable to readers and allowing writers to emphasize other differences without technology in the way.
There are an awful lot of fantasy settings that inhabit a weird realm that draw bits and pieces of mishmashed, mutated, and poorly understood versions of "Middle Ages" history. (Arda and Westeros are both excellent and well-known examples.) Trying to use historical processes to understand them is missing the point.