"Good enough" bridges still last 50+ years. We could design a bridge to last 200 years but we won't even know if the design we have today will even be needed in 200 years. Maybe by then we all use trains in underground tunnels.

Certainly, "enough" is doing a lot of work and things get blurry, but I think "good enough" is meant to capture some of that. Over building is also a problem. It isn't strictly true that building longer lived things is cheaper over time either, it obviously depends on the specific things getting compared. And if you go 100 years rather than 25 years, you'll have fewer chances to adjust and optimize for changes to the context, new technology, changing goals, or more efficient (including cost saving) methods.

Obviously, there's a way to do both poorly too. We can make expensive things that don't last. I think a large chunk of gripes about things that don't last are really about (1) not getting the upside of the tradeoff, cheaper (in both senses) more flexible solutions, and (2) just getting bad quality period.

It might very well be that building and maintaining a bridge for 100 years costs three or four times as much as building and maintaining one that last 50 years. If demolition costs are not same as cost of bridge well in long run replacing the bridge ever 50 years is cheaper.

On whole it is entirely reasonable optimisation problem. What is the best lifespan of single bridge over desired total lifespan.

Depends how much the infrastructure and needs around it changes.

But we also got roads and hospitals.