That makes sense. But wouldn't the "write quality software" and "test the software thoroughly" still be relevant to the individual pieces? If the chance of a

That makes sense. But wouldn't the "write quality software" and "test the software thoroughly" still be relevant to the individual pieces? If the chance of a catastrophic failure is the product of the failure rates of the pieces, getting P(PartFail) low helps too -- even if having multiple backups is the main source of protection.