MODERN COMMENTS IS A DISASTER for my work as an professor who has to read & annotate hundreds of pages of student papers in very short turn-around time-- it negatively impacts my work flow by adding extra steps and consequently time. Most of the "fixes" you've outlined were available in legacy comments and much less cumbersome to exercise. Some are critical issues with modern comments that make it infuriating; for example, the lack of clarity of what text matches which comments has lead to extra discussions with students to clarify the suggestions I make to improve their drafts; "Show all revisions inline" junks up the paper much more than the two different lists of revisions in the margin and comments outside the document.
My intent is not to "start a conversation" or collaborate on a document-- it's to give feedback that is accessible and easily managed. Being forced to use the "post comment" button means extra time for adding comments as well as having to reconstruct them when I forget to hit the button.
In other words, you've fixed something (legacy comments) that wasn't broken by replacing it with something that IS broken (modern comments) for a huge segment of your users.
I don't work in 365 AT ALL -- rather, I've purposely upgraded my perpetual Office suite over the years; however, since my university has implemented 365 across the boards, all Word docs (even those saved to my hard drive) will only open in the 365 app; my tech folks say they can't turn off modern comments just for some users -- it's all or no one. If I wanted to use a virtual platform for collaboration, honestly Google docs is easier. Now you've permanently turned off my ability to revert to legacy comments.
CUSTOMERS should have control and choice -- everyone's needs are not the same. We shouldn't be forced to use something that doesn't work for us as an individual user.