> If Bob commits to repo A and Sally commits to repo B, their commits can't result in a merge conflict.

This holds true when A and B are leaf repos, but gets tricky with repos inside a dependency graph. More concretely, if C depends on both A and B, and it turns out that C depends on A and B in such a way that A_bob and B_sally are mutually incompatible, you need some kind of mechanism for reconciling that.

Of course, exactly as you point out, mono and multi are two tradeoffs for the problems that large codebases intrinsically are.