I want to piggy-back on DrGHunt's excellent commentary about the legal profession by adding another particularly large elephant in the living room - federal government, at least the US variety. I recently retired after 15+ years in federal government communications and I am confident that "Modern Comments" would be an unmitigated disaster in many agencies. In my agency, we use an approval process called "clearance", a clearly defined (one might say rigid) structure where documents move up the ladder from lower departments to leadership. Some collaborations occur along the way, but most of the time documents are edited and suggestions implemented at the team/division level, then "cleared" versions are forwarded up the food chain, often through four or more levels. This is far from a perfect process (version control anyone?) but for large organizations with clearly defined hierarchies, this is how it works.
The answer, which Microsoft seems to now understand, is not to force everyone into the Google collaborative model, but to offer the "Legacy" model for those who prefer it, and have spent years building personal and organizational work processes around it.