... Two code paths (old&new) - I've been doing this to some extent without trying to get any insight, or make it a practice (of my own first), but often the old code just stays before deploying the newer (day or two), or before submission, and then it's killed (or #ifdef-out).

I liked the "empathy" bit though. Recently I've did the UCLA Extension TMP courses, and one of the lecturers - Jorge Cherbosque, PhD talked exactly about this and much more - here is the lecture (highly recommended)

https://www.uclaextension.edu/tmp/Pages/79th/D1.aspx

Is there a video behind the talk?