One thing to consider is that monorepo tooling is (outside of Google) still pretty immature.

At Square, we have one main Java "monorepo", one Go "monorepo", and a bunch of Ruby repos. The Java repo is the largest, by a huge factor (10x the Go repo, for example).

The Java repo is already noticeably slow when doing normal git operations. And I'm told we've seen nothing of the pain the folks at Twitter have had with git: their monorepo is huge.

We periodically check to see how the state of Mercurial and Git is progressing for monorepo-style development. Seems like Facebook has been doing interesting stuff with Mercurial.

But I still miss the fantastic tooling Google has internally. It really is so much better than anything else I've seen outside.