The incentives of individual praise aside, engineering teams tend to gravitate toward full rewrites because they incorrectly think of old systems as specs. They assume that since an old system works, all technical challenges and possible problems have been settled. The risks have been eliminated! They can add more features to the new system or make changes to the underlying architecture without worry. Either they do not perceive the ambiguity these changes introduce or they see such ambiguity positively, imagining only gains in performance and the potential for greater innovation.
