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.

Yes, exactly. A successor could theoretically replace Word, but first it needs to replicate all of its existing functionality.

For a competitor to supplant Word, it would need to:

- Be fully backwards compatible with .docx. Lawyers will inevitably receive .docx files from counterparties that they need to review, redline, and mark up. The new processor has to handle everything Word does flawlessly. (As an engineer who has spent considerable time building a high-quality docx comparison engine, I can tell you this is tremendously difficult.)

- If it introduces a new file format, support seamless comparison and conversion between that format and .docx. Not technically impossible, but also tremendously difficult with marginal upside.

- Defeat the Microsoft Office bundle in the market — meaning it either offers enough advantage that organizations pay for both, or it replaces Excel, PowerPoint, and Outlook too.

Given the enormous challenge of building a viable Word competitor and the marginal room for improvement that Microsoft has left on the table, I think it's very unlikely that a competitor will threaten its market position.