You are assuming that the observed tool strain scales with the number of developers. In my experience it scales with the number of coupled concerns inside the same repo. Now, this may be somewhat correlated with the number of developers, but not entirely. Therefore in my experience again you can end up with a moderately sized company running into tool limits with a monorepo. FB doesn't have those problems because they use different tools.
> FB doesn't have those problems because they use different tools.
Exactly - instead of using microservice-oriented tools, they use tools organized around monoliths. And that decision serves them well. That's the whole point.
At the average company, standard tools will work fine, while companies with large engineering teams have the resources to maintain custom tooling.