For anyone interested in why monorepo works, I'd recommend the book Software Engineering at Google: Lessons Learned from Programming Over Time. It has detailed the reasons for the One Version Rule and Version Control over Dependency Management.
Doesn't Google use Perforce though, which (last time I used it) forces a monorepo approach? git doesn't have equivalents to branchspecs and clientspecs.
It is because Google wants a monorepo, then Google choose to use Perforce (and later Piper). It is not that Google uses Perforce and thus are limited to a monorepo.
The core value behind monorepo (and monorepo-like approaches) explained in the book is that dependency management is harder than version control.
