It’s always possible for a civilization to regress. Look at what happened to Europe during the so-called Dark Ages.
I mean, how could you lose the formula for the Roman invention of concrete?
Isn’t that the problem with human history? We never know the great ideas that were once common knowledge, but were lost as tribes, nations, and people were decimated and extinguished. The cause might have been disease, famine, war, natural calamities, or sheer vitiation of the genetic line.
But once the wise ones are gone, their knowledge is gone with them.
“How the Irish Saved Civilization” is a great book on this very point. But for the geographical isolation and absolute determination of a small, consecrated band of brothers, most of the knowledge and philosophy of antiquity would have been lost forever.