Yes, that's an entirely different thing. But I would rather compare a production software system to something bit more dynamic, like a nuclear plant or an airplane than a bridge.
This is just survivor bias at work, though. How has road building been in the past 50 years? If your road has seen heavy use, it has massively changed.
So, then should we assume that software is like road building? No, rather I would assert that there are some pieces of the road that are more isolated from dangerous use and changes than others. In software, we have some of that, too, but not to nearly the extent. We are often holding up pitchforks to rewrite everything.
Given a Fortran compiler I see no reason why lot of the numerical stuff in netlib would not work just as well in 60 years time: http://www.netlib.org