Mono vs poly at scale needs resources. You have different compromises with each and so the resources go to different places. However there is no clear cut winner despite a few mono repo at scale advocates trying to claim otherwise - they are always completely ignoring the issues with a monorepo setup.

Multirepo at scale also needs resources, there's an enormous amount of work required for version bumping and synchronizing everything. People always completely ignore all those chores. Generally, you have either a massive amount of tech debt or you have one person doing nothing but running around doing all that work for everyone else (works great if all that work seems to magically appear for you). You can also be furiously working at automating all those chores, but that's the same level of effort you'd have to throw at scaling out a monorepo--just different.

Exactly because monorepos have least overhead when they’re small, monorepos generally win because you need to be small for a long while until you get big.

By the time you’re “at scale” (who knows), and all these monorepo at scale problems start to overwhelm, you can switch strategy, because the economy of polyrepos is so obvious by then.

So far, I’ve started a new job a handful of times by collapsing a premature polyrepo strategy: people were not experienced enough to merge two git repos without a common root.

I’ve only once went the other way, and it incurred so much overhead, it decreased developer productivity by some small but not insignificant percentage.

To be clear: I’m not a maximalist. All of my open-source work is exceedingly compartmentalised. My DNS library is separate from my external-dns webhook is separate from my fork of external-dns. They could all live in one repo. But FOSS encourages reusability, commercial software encourages clumping and vendoring.