Both variants are nasty in a huge repo. The quoted example at the end is a good example. If you have different repos, you sometimes can't push to repoA because you are depending on repoC. If everything is in one repo it should just work. But that's not really the case. You still need to wait for changeC before you can go and do changeA. Same problem, independent of the repo situation.

The solution is that changeA must be backwards compatible. In a complex system you always need to have some kind of backwards compatibility, at least for some time.

In the end both (mono and multi repos) doesn't really work in huge, complex scenarios.