It's something the US is dealing with, particularly in military and aerospace. You stop doing something long enough and, despite having schematics and documentation around, nobody is left that actually knows how to do it. See for example the retirement of the Space Shuttle and the resumption of classical capsule-based spacecraft, we're still in the process of re-learning a lot of knowledge we already gained during the Apollo era. And that's merely a few decades.

Not to mention "dark ages" aren't necessarily the loss of knowledge, but rather the loss of governance. Neither China or Europe ever lost the ability to build roads, but yet the infrastructure deteriorated and was not replaced. This suggests economic, political, and/or religious causes, not the loss of knowledge.

I don't think we've figured this governance thing out to the point where we're safe from regressing to a state where public infrastructure breaks down in a massive way.