> It might have been difficult to procure pozzolana ash when the trade routes were less safe in the early Middle Ages, which would make durable concrete rarer.

I think this could be how modern technology will be lost in the future. We have been seeing a reversal of globalization. We have seen countries guard their raw materials more tightly than before. Trade routes could disappear for geopolitical reasons. The United States was happy using Russian RD-180 rocket engines and Russian uranium after the Cold War; and look at how quickly this trade has become undesirable. Now consider materials and technologies lesser known than uranium or rocket engines, or simply less critical to a country's military might. A country might not even know it has a single sourced component until trade has stopped.