Labor was free because of slavery, so the economics were not the same. Current engineering has the concept of “over engineering” which is what cracked-up addicts in wall street call “building to last”, due to the “expense” of not being shortsighted on a quarter by quarter basis.
From my pov I think you’re repeating exactly what I said, but I appreciate the additional details.
My last point is that “the design life we choose” is usually dictated by non-engineering forces. A 12th century king can throw resources at a problem. A 20th century governor cannot, and doesn’t care to. They care about the bridge lasting until the end of their term limits.