The real issue here is that the middle ages as a field of research tries to knit together very different trends isolated from one another by the Carolingian period. The early part that can rightly be called the dark ages sees depopulation and population replacement in peripherical regions from Britain to the Balkans. Contrary to popular narrative one of the biggest losers of the dark ages was Christianity itself as it lost ground to both Islamic arabs and pagan slavs and steppe newcomers. Pic related is something like the nadir of Christianity with a lot of former late roman territories lost most of said losses to be islamized and never regained and while slavs would eventually be converted this was not what someone living at the time would have predicted.
However, the vast majority of medieval historians focus on Carolingian and post Carolingian typically called High and Low middle ages. The early periods before Carolingian times are understudied and Late Antiquity historians typically come closest to it but they always end up focusing on the eastern mediterranean.
Then halfassed mass schooling takes in and has the average joe mix it all into a 1000 years dark ages followed by renaissance and enlightenment, leaving fertile ground for contrarian takes such as pitting roman period versus medieval period history which makes near zero sense if you try to get into post Carolingian history in many of these places given the obvious cultural continuity especially through Christianity.