Thanks for sharing that. It's mind-blowing that Google runs everything from a single repo. Most places that I work with create several repos for just one project.
Yeah, it was a significant infrastructure cost. I heard at one time that the single largest computer at Google was the Perforce server. They ended up completely re-writing it (called "Piper") for scaling.
This is sort of what I was alluding to with my comment about Google having different concerns than many other companies. They can afford to dedicate a number of engineers to maintaining a monorepo system and then re-writing when it doesn't scale. That said, I personally believe that there are a lot of benefits to monorepos, and I think those tradeoffs are worth it for other companies too.