Ok, I see what you mean. To me this sounds like you not taking a simplified example for what it is. In a perfect system all you say can be quantified down into

Ok, I see what you mean. To me this sounds like you not taking a simplified example for what it is. In a perfect system all you say can be quantified down into money, and be priced in when comparing 25 and 50 year bridges. In the real world there are other factors too, like the risk that the nearby city will grow enough that the bridge will need to be expanded after 25 years. The entire second half of the 50 year cost may be wasted in that case, as expanding the bridge may be at least as costly as building it in the first place. Or in 10 years traffic demand goes down, so that this bridge is no longer needed. Or boat traffic changes, and this bridge is now too low. More waste to tear down the 50 year bridge than the 25 year bridge. Or if the bridge gets bombed after 25 years. Then too the 50 year bridge is a waste.