This is a valid observation; however, we also need to consider that we write orders of magnitude more software in 2018 than we did in 1968. I don't think the defect / LOC (or whatever metric you want to use) has decreased. So while the cost per defect might be far lower, the total number of defects keeps increasing. The mitigation strategies that I've seen reduce costs mainly because they simply choose not to fix a lot of problems.