Would there, though? I mean, we HN readers want that, but I don't see any evidence that the average Office user is clamoring for it. Even comparatively crude systems like Track Changes aren't used in my experience as often as they could be, and those are a hundred times easier for non-technical people to wrap their minds around than a Git-like system would be.

I suspect that part of the reason this problem is so intractable is that it's really two problems. The first is actually tracking the changes to the underlying document, which, as you note, is hard enough by itself. But then on top of that is the second problem, which is that you'd also have to come up with an interface that could make the power of a Git-like system comprehensible to non-technical users. And you'd need to solve both problems to have a product that would actually be ready to take to market.