This is (good BTW) "pure financial theory".

But the assumption here is that the replacement bridge is built "overnight" or however without altering in any way the normal traffic on and around it, which is very rarely the case, more likely you should expect months or years (of course depending on the size/complexity of the bridge) of limitations/difficulties with the city traffic, noise, dust and all the issues usually connected with construction sites.

This is an indirect (hidden) cost of all kind of infrastructures that the population/users will have to bear.