I guess that I'm saying that eventually code, reused and added to in even the most careful way, becomes more apt to failure than a rewrite would be. The original citing here was the example of the code written for the space shuttle, and how it has been reused through the years, even in the face of what has to be changing requirements. Yes, it works, and perhaps in man-rated tasks where safety is first, that's supremely important, but most computer programs aren't man-rated. Overstressing conservatism is not always the best way.