it might be true that Google or Facebook has some monorepo alive since more than a decade or two decades ago, however, that doesn’t means it’s the right approach for any new projects to adapt in 2019.
tech has been improving on loose coupling and SoC, which is a design idea since GOF and even earlier. this has continuously generating the benefits from version control, project segregation, building, CI/CD and micro services, which resulted faster and better quality delivery and long term maintenance. I will be surprised if monorepo would flourish in next decade or any near future.
quite interesting to see all those ideas/noises/debates come back and forth though.