On the historical argument, I'd say that comparing the in-game term 'Dark Age' to the period in history that used to be known as 'the Dark Ages' is perhaps not very helpful, given that this terminology is considered somewhat out of date.
From the same Wikipedia link:
To me, all this means is that we're not looking at the Early Middle Ages as an exemplar for the sort of 'Dark Age' the game is trying to convey. Saying that the Early Middle Ages weren't really a 'Dark Age' doesn't mean that the concept cannot exist at all, or that there weren't plenty of other difficult and challenging times faced by historical civilisations that the in-game 'Dark Age' could be representing.
As I said earlier, I consider myself a casual player. I don't really care about Dark Ages being especially difficult. I just want them to feel like something you want to avoid, or that going for the Dark-Heroic slingshot should pose some kind of short-term challenge or slowdown as a tradeoff.
The point that the game should use (again, IMHO) is that "Dark Ages" were never Entirely Dark. In fact, if you divide up consequences into Political, Diplomatic, Religious, Scientific, Trade, Production (the usual suspects in Game Design) you will be hard put to find ANY period in history when all of them simultaneously went into 'decline'.
In fact, to use a different example than the Early Middle Ages, the "Greek Dark Ages" were supposed to be the period between the collapse of Mycenean societies about 1100 BCE and the "Archaic" (beginning of the Classical City States) period around 800 BCE. In those 3 centuries the population declined, writing disappeared, the political organization of the Palace cultures disappeared, most long distance trade disappeared. All Bad. But Iron-working was introduced generally, resulting in a complete change in military (iron was cheap enough that many more men could be armed than just the elite nobility) and they started colonizing nearby places like Cyprus and did keep some trade going throughout the period with the Middle East. In other words, it was actually much worse than even the traditional view of the Early Middle Ages - where at least the forms of Roman administration and governance were maintained, at least locally - but even so it wasn't totally negative in all respects. And, also significant, the following period (800 - 600 BCE) was the High Point of Greek colonization - the earliest city states saw a population explosion that was exported to new Greek cities all across the Mediterranean, North Africa, and the coasts of the Black Sea and Anatolia. The only comparable expansion of a group I know of would be the expansion of the population of the USA into the midwest and west between 1790 and 1890 CE.
So, "Dark Ages", "Times of Troubles", "The Warring States" - names all over the world refer to what we are talking about here - should never be 100% negative (that's bad both historically and for game play) but in all cases seem to mean primarily a collapse of central Political authority - the 'Free Cities' mechanic of Civ VI expanded dramatically. That is also almost always accompanied by a loss of long distance Trade and money/Gold and authority for what government remains, but not always negative results to individual cities and areas within the Civ: even during the 'traditional Dark Ages' of 600 - 1000 CE in Europe, while cities stagnated or got smaller, the total population of the region around them does not seem to have always lost population, and the prosperity of individual hamlets and villages may have gone up (just for an instance, the numbers of water and other Mills and access to iron tools from local blacksmiths seems to have gone up dramatically in that period compared to the previous Roman Empire/Classical Era).
As others have stated here, we need a lot more nuanced approach to the 'Cycles' of Civ-Building in the game, as expressed in Golden, Dark, Heroic, or other 'Ages' or Eras.
