I think you should clarify your message a little bit because Google Drive works well in mobile (and offline), too, especially since they started giving away Quick Office to augment their "normal" Drive app. Additionally, most of your features are available in Drive/Docs, but sometimes just executed differently (like diffs, presence and notifications). I can absolutely see a value in the Quip way of doing things, but not for a large enterprise with high volume and complex documents, not to mention the common use case of taking a native binary doc of some sort and uploading+converting to Drive format. The SMB use cases you use as examples are spot-on, but don't mesh with my needs as an administrator of a 20,000 person domain with 800,000 documents online. I also wouldn't pay $12/user/month for Quip when I could get all of the rest of Google Apps for only $50ish.
Another question I'd have is what your long term plan is. The last thing I need for my business (of any size) is to select a vendor for a productivity tool and have it disappear due to acquisition, pivot, or bankruptcy in a couple/few years. (Coincidentally, I just saw the HN post announcing this: https://catch.com/)
Speaking of diffs, here's a FR that would probably be easy for you to implement, and one that a lot of Google Enterprise customers have been complaining about for years. It's possible to view and revert to previous revisions of Google Docs no problem (although the UI isn't particularly nice), but it's not possible to freeze versions of a doc. The method most people seem to go about this currently is to save a copy of the frozen version to docx/pdf and place it in the same folder as the working copy, but this is incredibly annoying.
Regarding commenting and collaborating on docs, one of the really nice features Drive has that you might consider adopting is their method of handling comment threads, and the integration with email (they all @mentions, too, using the +username syntax, btw).