I don't agree that economic collapse is the correct term as it suggests total production drops alongside things like hyperinflation etc.
Rome allowed for an incredible amount of wealth transfer, but the total economic output of Europe at say 500CE was not significantly lower than the total economic output of Eurpoe at say 200CE.
Watermills were developed by the Romans, but continued to improve though the middle ages. Iron smelting has a long history, but was improved though the middle ages. You can look at a huge range of productivity increases throughout the middle ages as yields improved due to constant innovation.
Lead refining collapsed after Rome's crisis of the third century. There are Greenland ice cores saying so. Lead production didn't recover to Roman levels until the 16th century.
A few individual technologies progressed through the middle ages (but noticably not during the dark ages in the west), but social organization to build large scale projects is also technology, and that technology was utterly lost for a thousand years.
Rome had what we would recognize as Adam-Smith-like industry, a market economy, long distance trade, and mass production. It was a surprisingly modern world, and it collapsed. We're basically the second modern world, and we can collapse too.
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.
Rome allowed for an incredible amount of wealth transfer, but the total economic output of Europe at say 500CE was not significantly lower than the total economic output of Eurpoe at say 200CE.
Watermills were developed by the Romans, but continued to improve though the middle ages. Iron smelting has a long history, but was improved though the middle ages. You can look at a huge range of productivity increases throughout the middle ages as yields improved due to constant innovation.