It's tolerant of random failure of individual components, yes, but the entire spar could fail under an overload condition. For this failure mode, the only way to ensure a suitably low failure rate is by setting an appropriate safety factor.

> but the entire spar could fail under an overload condition.

Each component individually is designed at 150% of the maximum load ever expected.

The spar has redundant components. Any part of the spar can crack all the way through, and it will still fly safely.

Redundancy protects against some failure modes (e.g. unrevealed fatigue cracking) but not overload, which is a common-mode failure that doesn't care about redundancy if the load is high enough. It becomes a matter of "probability of exceedance".

Electrical/mechanical systems are different and can usually be separated/segregated etc, but there is only one structure.

There was a famous crash where the pilot flew through some wake turbulence and caused the tail to fall off by improper rudder inputs. at a certain point there is only one of something.

The rudder structure is redundant as well. That particular accident was caused by unexpectedly high loads on the rudder, not a lack of redundancy.