And all bridges do last roughly 50 years. Look at all bridges and figure out the average, median, or mode lifespan is reasonably close to 50 years +/- depends on where you draw the lines.
A tiny fraction reach 500 and so far none that we know of have hit 5,000 though there’s a few possibilities.
> if tomorrow we decided to build a new bridge next to every old bridge, average lifespan would half.
The only way building a bunch of new bridges changes lifespan is if we changed how they where built.
Figuring out lifespan rather than age is at best an approximation after looking at the bridges that didn't survive and the condition of bridges that do, but the uncertainty is low.
A tiny fraction reach 500 and so far none that we know of have hit 5,000 though there’s a few possibilities.