You're right that monorepos are just fine for small company. The problem is that at some point they don't scale without code review discipline and sophisticated tooling (such as Google's); thus there is a bottleneck where it becomes harder and harder to scale it until you get your tools right.
Yes, but that point is probably when you have hundreds of engineers, at which point you can afford to have (and probably will already have) a few engineers working on internal tooling.
