I don't necessarily think the knowledge is lost, but the second system effect sure is in full swing.
I remember picking through an aerospace scrapyard in North Hollywood a decade ago with extremely-talented launch engineers (and entrepreneurs). The aim was to look at parts, measure them and figure out why they were built like they were. We looked, a little, at stuff like nozzles. But mostly we focussed on bolts, joiners, turbine blades and the like.
Down to the fact that the Eastern Roman Empire was going strong.
Even in 2014, when I was in North Hollywood, SpaceX was carrying the torch.