More realistically, the "dark ages" were a time of political dislocation and fragmentation of trading networks, so Europe just got a lot poorer. Rome had knit together a vast trading network, but without Roman armies you couldn't trade long distances anymore.
You forget how to build Roman roads when you don't have a government around to build roads, and even if you did, you don't have the money or manpower anyway.
Medieval Europe invented a heavy plow, which the Romans never came up with, so it was not all disintegration.
https://sciencenordic.com/agriculture-agriculture--fisheries...
> Largely unaffected from the turbulent political events following the demise of the Western Roman Empire, the importance of watermilling continued to grow under the new Germanic lords. (...) By Carolingian times, references to watermills in the Frankish Realm had become "innumerable".
[1] https://en.wikipedia.org/wiki/List_of_early_medieval_watermi...
My hope is that I am as ignorant about our society's capacity to bootstrap important technologies, as I am in my own capacity to do so.
Not a referral link