This is not so much a matter of safety factor than redundancy. Safety factor = "it only fails if the bridge is full of heavy trucks and they all are overloaded

This is not so much a matter of safety factor than redundancy. Safety factor = "it only fails if the bridge is full of heavy trucks and they all are overloaded 2X" Redundancy = "we have 6 stays and any single one can snap and the bridge will stay intact" Airplane wings cant have either. Because safety factor higher than 1,2 would make the plane too heavy to fly. And redundancy from multiple wings would make the system aerodynamically so unstable that snapping a single wing would still cause a crash. In those cases you need to have very good testing program before installing the structure. And then you need a good inspection program to protect against fatigue and corrosion. The latter was missing in this case.