We can't figure out how the Egyptians (or Aztecs or Maya) built their pyramids. Stonehenge is an engineering mystery. The heads on Easter Island are likewise a riddle to moderns.

Indeed, it's well-documented that we have no idea how to reproduce a Saturn V booster at this point -- too much documentation, expertise, and manpower have been lost in the intervening years.

I'd reckon that there's more knowledge and skills that are lost, intergenerationally, than we gain through R&D.