Remember, spacecrafts have weight constraints that limits redundancy options. Besides this was a 'tracing bullet' designed to test an unproven procedure before committing to a more expensive mission, so there's that.
Designing a reliable system that is failure resistant is what aerospace design is all about. With INS fail op design is the norm. Yes accidents still happen for common mode failures, but those are supposed to be worked out before committing hundreds of millions of euros.
Costly mistakes happen when people take unjustified shortcuts. Like not testing Hubble's mirror on the ground, and ending up with a myopic telescope in orbit that then needs another billion dollar mission to correct.
