[WalterBright]

> 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.

[HPsquared]

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.

[weaksauce]

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.

[WalterBright]

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