In early Imperial Rome prestigious families would fund public works as a public symbol of their wealth and power. This shifted over time to a more self-interested display of wealth and power in the family's villas. If the aqueducts and roads have existed for centuries in your region there is less opportunity for those public displays of civil engineering and I would suspect the skills and knowledge did wither though I can't say to what extent. Look how fast the knowledge to build the space-race era rockets was lost in the US, and that's with modern record keeping and no major domestic disasters or upheavals.