Google Sheets lets you chat with other people actively using the sheet, but it has no history and overall feels like the simplest thing that could work.
Slack, by contrast, seems like everything you could ever want in a chat client (and probably several things you don't want). It seems like there should be a way to embed Slack into the document (possibly using Google Apps?), but I don't see any straightforward way.
Integration with anything else would also be a big step forward.