Since you’re citing fiction, have you read the foundation series? The original inspiration for that book was The Decline and Fall of the Roman Empire, which is

Since you’re citing fiction, have you read the foundation series? The original inspiration for that book was The Decline and Fall of the Roman Empire, which is a history book (arguably the first modern history book). Anyways, that just to say that people in post-Roman Empire Europe lived out the reality of lost knowledge. They lived with the remains of incredible art, architecture like the coliseum, public works like the aqueduct, etc. But they wouldn’t have known how to reproduce those works. How to make concrete, etc. was knowledge that was lost to them.