I think Knuth originally envisioned a writer working with a book designer/TeX developer to build a new set of macros for each project, and once the book was wri

I think Knuth originally envisioned a writer working with a book designer/TeX developer to build a new set of macros for each project, and once the book was written the macros would be left alone. Therefore it would be very important to avoid breaking old macros, meaning TeX's behaviour needed to be nailed down. Today we would more likely take a PDF-like approach, specifying the file formats and using multiple programs to generate and process them.