So you've introduced the context of large scale mission critical software. Not all software is large scale and not all software is mission critical. Sometimes

So you've introduced the context of large scale mission critical software. Not all software is large scale and not all software is mission critical. Sometimes a crappy script that saves an administrator 2 hours a day can be a Huge win. Sometimes lives are on the line and bugs are not acceptable. Other times the cost of a bug is some internal user has to deal with a little frustration. All investments are about weighing the cost with the potential benefits. Developing software is an investment.