Given that the example used was pikemen, this is a bit suspect, to put it mildly. Peasants didn't compose armies until the French revolution, by the way. (Exception for the Swiss here.)
I was using peasant in the "anybody who wasn't a part of the aristocracy" who while excluded from the low level skirmishing endemic in the "Dark Ages" definitely were levied to fight in larger wars.
But you are correct in the sense that Mongol armies generally made mincemeat of the average European, knight-led army. But, and this is a big but, that was because of a Mongol military innovation.
I never said a thing about the Mongols.
Well, unless they venture into uncharted territory, like Germania or Parthia.
The Romans never had a serious German or Parthian problem, but the Germans and Parthians had a serious Roman problem.
Trajan and Septimus Severus conquered the Mesopotamia heartlands and established Roman provinces there; while in retribution for Teutoberg Forest, the legions did an admirable job of burning, raping, and pillaging German towns. Against the Parthians and Germans the Roman Army performed very well. That they lost a battle or two sort of proves my point: that no matter how much you stack the deck in your favor Lady Fortune is a fickle mistress.
Military proficiency is such an elementary part of a state's survival, that it is surprising it got so much worse.
Different != worse. Different states had different goals with different resources with different understandings about what warfare entailed and how to go about doing it. The style of warfare and societal organization employed by Aethelstan was well suited to how the Anglo-Saxon kings understood the relation between warfare and society. Warfare was largely about getting prestige and loot to curry favor and distribute benefits to your followers so it makes no sense to try and support a large "professional" army which would only introduce more possible competitors for royal favor.
But a 6th Century Lombard Lord should have known some of it.
Not fighting with a professional army did nothing to stop the Lombards from carving out a large, prosperous, and stable kingdom despite the efforts of the "professional" Roman Army and the Romano-Gothic freemen levy/warrior aristocracy Ostrogothic Kingdom.
I'm guessing that some of it was due to technology (the ability to mass produce chain mail for example), but from what I gather late medieval technology had surpassed classical Rome in most respects.
Where do you get the idea that chain mail was ever "mass produced" in the modern sense of the word or that it was somehow lost due to technological regression? Chain mail fabrication was, is, and always will be a tricky, fiddly, and time consuming effort. Dark Age kingdoms were not poor. A couple of years ago a large collection of Mercian gold inlaid sword hilts was found buried in the UK. Based off of our understanding of the size and structure of the Mercian army; either a royal officer managed to loose a quarter of the sword hilts of the Mercian kingdom in one go, or the Mercian army -and Kingdom- was wealthier than is commonly thought for a "dark age" kingdom.