In Britain, knowledge of building in stone completely disappeared. French stonemasons had to be imported to build the first stone Anglo-Saxon churches.
Roman concrete is really remarkable because it achieves a tensile strength equal to reinforced modern concrete without the Achilles heel of rusting rebar. It's also self-healing, meaning cracks exposed to water generate new crystals that seal the crack and re-bind the two halves.
It is very labor intensive, because it's too thick to pour. It's a thick paste that has to be rammed into the form.
Large populations with intense specialization allows all sorts of things you can’t do when people make their own shoes and grow their own food. Civilization took a huge “pause” after the Roman Empire fell.
Imagine the things we have today that we wouldn’t have if, say, a large portion of people are killed, by, say, a bad vaccine. Steel products. Cars. Gasoline. Cell phones. Kill off half of the world population, as numerous enviro-nuts and liberals want, and suddenly lots of things we take for granted just vanish. Coffee in norther climes. Cheap transportation. Electric power...just imagine the infrastructure required for that...
Our civilization is fragile. And, most of us wouldn’t recognize food in the wild. If it doesn’t grow naturally on a Styrofoam tray we’d starve.