Except it doesn't, really, scale well in practice. Even now, writing and maintaining large documents in Word is typically an exercise in frustration. Don't get me wrong, there are many things it does well. Books aren't one of them, and neither is technical content (e.g. math) or other complicate typesetting and structuring.
This may just be an implication of mixing typesetting and writing tasks.
Are there any platforms where writing and maintaing large documents is not an exercise in frustration? LaTeX is great but it's not what most people would consider "easy." From my observations, non-technical people don't get Markdown, reST, or other markup-based formatting at all.
Is maintenance of large documents just an "essentially" complex problem, to use Brooks' classification?
I think it probably is "essentially" complex, but that doesn't mean that all tools are equivalent. Sometimes the issue isn't making something "easy", it's making it possible. Sometimes with Word you end up working your project around the tool because anything else is impractical - but that's exactly backwards from how it should be done.
This may just be an implication of mixing typesetting and writing tasks.