And yet, the fact that a bridge design contains a pillar that any expert expects to collapse after 30 years might tell you something about the designer's competence and thus about the viability of the whole design.
This is not so much about showing how to break their system, but about showing how their design methodology is likely to produce an unreliable system - because that (a) is much easier to do than actually breaking a system and (b) precisely because of that is how cryptographers usually work and (c) it is actually known to be possible to build systems that are more likely to be and to stay secure, so there is no point in compromising reliability for implementability.
This is not so much about showing how to break their system, but about showing how their design methodology is likely to produce an unreliable system - because that (a) is much easier to do than actually breaking a system and (b) precisely because of that is how cryptographers usually work and (c) it is actually known to be possible to build systems that are more likely to be and to stay secure, so there is no point in compromising reliability for implementability.