Nothing prevents you from using Emacs, or Vi(m), write in Markdown, Org or whatever and deliver a DOCX.
The amazing thing is that you can even work with people that use Word track changes, while staying in Emacs, Vi(m) or any other text editor and using Pandoc.
But, using a plain text editor and Pandoc for conversion is a lot like using Scrivener and "compiling" to DOCX: it's a one-way process. If your editor gets back to you with a marked-up Word file, you're going to need to address those comments/revisions in Word. (Or Pages or LibreOffice or something else that can round-trip those.) And from that point on, keeping your original "source document" and the Word document in sync becomes a bit of a juggling act.
When it comes to producing text for humans to read, there's a tool that virtually everybody uses and that's Word. You will simplify your life considerably by using it, too.
In fact, all documents I typeset with Pandoc -> DOCX look like really carefully typeset documents on Word because the style is so consistent and in line with what latest versions of Word encourage: separation of content and presentation.
But if you discover a good text editor, be it Emacs or Vi or something else, it changes the way you edit text. There are things that a good text editor does exceedingly well that are painfully hard - if even possible at all! - in a program like Word. Some of these are specific to programming, but some are specific to editing text -- Which kind of is what writers do, isn't it? ;-) -- and once you crossed that barrier, going back to Word - useful as it may be - is like going back to chiseling your writing into slabs of rock.