I don't necessarily disagree with you, but I did want to point out that a big part of what made it possible for Word to displace WordPerfect in the legal world was, literally, the fact that Word implemented full support for WordPerfect's file format including all sorts of weird quirky edge cases.
So, an analogous "Word-killer" today would presumably have to implement all of the docx format's weird quirks etc. On the one hand, the file format is standardized and open, so in principle that should be possible; on the other hand, it's a pretty gnarly file format, with a lot of nooks and crannies. Ironically, I remember hearing once that some of the weirder nooks and crannies of the docx format have their roots in... Word's WordPerfect interoperability features.
And as somebody who recently spent far more time than he expected to trying to reliably get data _out_ of a set of mildly-complicated docx files, I can report that the various fiddly details that the OP notes as being particularly important in the legal domain --- very specific details of paragraph formatting, complex table structures, etc. --- are a huge PITA to deal with when working with the docx format.