Here is what I am trying to say. We have learned a lot about building bridges over the years. Both with computer modeling and experience we understand now better how systems work. I guess obsolete is not the word so much as, not build with the redundancies modern bridges are built with. We also have a better technology.
For example, bridges in earthquake zones now have their rebar bent in a “cork screw” fashion (forgive me if I get this wrong because I am remembering a Science channel TV shows from years ago. It was one of the things we learned from the big earthquake out in California during the 1990s.
It isn’t that the engineering then was bad, it just that we have 40 years more experience and a whole host of tools that they did not have.
The point here is we, in Minnesota, could of upgraded that bridge any time since 1990 but choose instead to spend our “infrastructure” dollars on new projects and gimmicks. The problem here is not that the Government doesn't have enough money, but that it miss spends too much of the money it does have.
I see your point, however it is 40 years in a 2000 year old art.
We build silly bridges that load concrete in flexure (big horizontal beam, with the top section loaded in tension) Concrete is terrible in tension, it likes compression. So, we add steel tension cables to preload the span in compression. This works and keeps the entire cross section in compression. All is good until the steel fails (rust). The only reason we do this is architects hate arches. ...according to a CE friend of mine that builds bridges.
About the only thing we’ve added since the 60s is cost and some prefab capability.