My theory is that doing so would cannibalize their user base.

I think that those who buy a pro camera nowadays do it because they care about photography itself. For that public post-processing is a touchy subject: besides the philosophical aspect of "is this a true representation of the moment I captured?", there's a real chance that their work will be under-appreciated or rejected altogether if their camera is known as "the one that retouches your photo automatically".

I'm not sure that Nikon's sales would improve that much if they offered auto-post-processing (is that something amateurs a tively want?), but I can imagine that their current customers would be unhappy.