If it doesn't interoperate perfectly with Word's Track Changes, that doesn't matter: your counterparty in any legal negotiation is sure to be using Word.

I wonder how hard it is to interoperate perfectly with Word's Track Changes, or, let's say, with a minimal set of critical Word features. Could a small smart team pull that off? That's how I'd try to do it if I were trying to kill Word. Or is Word so complicated that it would just be hopeless?

Track changes isn't part of any standard, so it's harder to duplicate.

Technical superiority won't defeat Word. Office is on every business desktop because of the licensing regime that Microsoft has been very successful at implementing.

Frankly, it's better to own the drafting process yourself : We're happy to show all the black-lined versions, but (due to 'technical difficulties') get our counterparties to send over manual changes which we implement...

In some cases (like the state office I work at), Libre Office track changes would be fine. We aren't all doing legal negotiating with counterparties (MS Office power users! I am super impressed!)