Imagine if during the height of the Greek golden age most of the largest cities were razed to the ground and their populations systematically exterminated. That means Athens as we knew it would have been reduced to rubble.

That's what happened during the Mongol Conquests. The fall of Baghdad alone caused at least several hundred thousand people to be killed. The main library, which had collected countless works over the centuries, was gone.

Baghdad was rebuilt, of course. Economically and politically it bounced back since it was still right on the silk road. But scientific study never really recovered there.