For what it's worth, I work on a project that is split into multiple repos that we now wish was a monorepo. Internal versioning issues are complex and end up eating a lot of time in our CI/CD workflow when they deliver no actual value to the end customer, since it all ships as one discrete unit. Ultimately we have a couple of FTEs that are basically hand-picking commits out of different repos to try to assemble a complete system of all versions that integrate with eachother. If we were working in one repo and kept breaking interface changes in feature branches this work would be significantly reduced, and the improved visibility of those cross-cutting changes would probably also motivate a faster turnaround time on them.

I would think that versioning and compatibility issues are the main driver of monorepos - if you are ultimately shipping one product, why get wrapped up in all the labor that can be involved in breaking it down while still being able to pull off working versions to test? Might be a much better decision to just treat it as one giant repo that always stays in a working state.

And finally, as a more soft-factors issue, I think that a monorepo can help to reduce siloization. We sometimes have issues with teams not liking it when people mess with "their" repo, which slows down cross-cutting changes by a huge factor. A monorepo would probably be a powerful factor against this kind of thing.