... there's a difference between having appropriate test case coverage and accounting for every single possible failure. This is where the old "Perfect is the enemy of Good" adage comes into play.
Absolutely, and I'm not disputing your opinion or the old adage.
I'm simply advocating taking software testing as far as it can reasonably be taken; all too frequently, testing isn't taken very far. Ditto for the thought put into software design.
It sounds as if we're saying the same thing and just talking past one another. In any event, I believe too much time is wasted in writing software, and in picking up the pieces when it fails - and not enough in up-front design and back-end testing.
Since we're bandying about old adages, here's a paraphrased one for you: if you love software and sausages, you shouldn't watch either being made.