I give it 2 years. Maybe only 18 months. I don't even know how many different (and confusing!) chat applications Google has launched already. Seems like their engineers, when they get bored, write a chat app.
My impression is that the monolithic repository only works because Google's built a bunch of custom tooling to support it. It doesn't mean any company can just roll all of their code into a single Git repository and call it a day.
The key here is CITC. All the source code lives "in the cloud" and pulls to your machine as it's needed (using a FUSE file system).