If you look at the last 400 years of bridge design you can see the transition from overbuilt compression structures to gossamer suspension bridges. This is a good thing, but it's easy to mistake old, overbuilt structures as being more "robust" when what they really are is an inefficient design meant to avoid failures due to a weak understanding of how the materials and structure will perform under use.
But you cannot ever convince me that our steel suspension bridges are going to last for centuries like our stone bridges from the 15th century and onwards.
By the 1940s, a stone bridge that might take 100 man-years of labor to create could be replaced by a bailey bridge that goes into place overnight. I'm sure if we wanted to create a bridge that lasts 300 years, we could dedicate more resources to it, but it would be grossly wasteful to do so - you don't know how long the current requirements are going to even be relevant. It's better to build a bridge that lasts for 50-60 years, and have the option value of replacing it with a better structure down the road.
And then we get to where we are now a lot faster. . .
"Over 90,000 miles of roads and 71,000 bridges are in dangerous disrepair, according to a 2010 U.S. PIRG study."