That may have been true a couple hundred years ago. It's not been true for a couple decades now, because budget became a constraint even more important than physics, and believe it or not, you will have to justify every dollar that goes into your safety margin. That's where the accuracy of modern techniques matter: the more accurate your calculations (and the more consistent inputs and processes builders employ), the less material you can use to get even closer to the designed safety margin. Accidentally making a bridge too safe means setting money on fire, and we can't have that.

That's the curse of progress. Better tools and techniques should allow to get more value - efficiency, safety, utility - for the same effort. Unfortunately, economic pressure makes companies opt for getting same or less[0] value for less effort. Civil engineering suffers from this just as much as software engineering does.

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[0] - Eventually asymptotically approaching the minimum legal quality standard.