Lead is a very poor proxy for economic development. It was actually known to be somewhat toxic though very useful even back then. And it's use declined in part because people got better at metallurgy not simply because the overall economy declined.
Flying buttress are an example of a significant and complex innovation in architecture developed soon after the fall of the Rome. Though it took a while to really develop. So, evidence for 'loss of knowledge' with building large structures is rather simplified. Loss of political and economic structures to build large scale projects is far more clear.
Rome at it's peak had ~70 million people across Europe, the middle east, and North Africa. Adding up all the individual peaces after it's fall presents a different story than looking at just Europe while ignoring the huge influx of goods and slaves from other territories.
Flying buttress are an example of a significant and complex innovation in architecture developed soon after the fall of the Rome. Though it took a while to really develop. So, evidence for 'loss of knowledge' with building large structures is rather simplified. Loss of political and economic structures to build large scale projects is far more clear.
Rome at it's peak had ~70 million people across Europe, the middle east, and North Africa. Adding up all the individual peaces after it's fall presents a different story than looking at just Europe while ignoring the huge influx of goods and slaves from other territories.