The problem is many people arguing for a mono repo feel like the size of their project means breaking into multiple repos mean put each function in a separate repository - which is obviously absurd. It is good if your project is that small, but you also are not facing the same pains as large projects are.
The reality is, at some point, having repos that are hundreds of GB with hundreds of active PRs also has its own downsides that requires tooling and workflows to combat. Meanwhile, splitting it all up introduces integration problems.
It's definitely pick your poison, though specific requirements and circumstances make specific paths more or less potent.