It will never happen because people will predictably abuse such bots in various gross ways, and the moment it leaks to the press, there will be a "think of the children" scandal.
I guarantee they focus-tested it and found that most people who remember Clippy don't have fond memories of him (and/or remember the memes/jokes about him more than they remember the actual Clippy), but, more importantly, most people don't remember him at all and wondered why this weird paper clip guy was talking. he hasn't been in an Office product since, what, 23 years ago or something? (not counting cameos)