Jon,
I convert 10-20 DOCX files to EPUB most every day. Because I know how to use Word, as I suspect does the OP, I never end up with a mess, particularly since Kovid implemented the DOCX conversion input plugin. And like the OP I segregate the text from the reusable templates, in Word its simple to apply different templates to the same text. The layout and typography is primarily inherited from tried and tested reusable templates that I've developed over the past 20 odd years.
On the odd occasion I have to go back to editing the DOCX its usually because of errors in content not layout or typography, and the odd glitches in the latter invariably stem from a trivial mistake I made in creating the DOCX - not from vagaries of conversion. The other day I applied a BigPrint template rather than the equivalent Standard template
Another reason an author will want to maintain the original as DOCX, ODT etc, is because that's where they'll start if they publish subsequent revisions. Non fiction often gets revised as new information comes to hand; maybe because some new GDR archives are opened up, or one or more judges reinterpret the Law in a 'novel' way.
Development and copy editors want WP files, as do most proofreaders. The idea that such people would be prepared to work with the text hidden in a sea of XHTML is akin to suggesting that in times past they would have been happy to use a Linotype machine.
The OP came to ask about one click multi output format conversions - where in any of your posts have you addressed that issue.
BR