> By the time you’re “at scale” (who knows), and all these monorepo at scale problems start to overwhelm, you can switch strategy, because the economy of polyrepos is so obvious by then.

Conversely to your experience, I have worked at a handful of places who have a monorepo that has been creaking under its own weight for years, but its structure as a monorepo now underpins the business, and so migration to a polyrepo simply never happens, and developers are now checking out a 50GB repo in its entirety periodically.