If there are multiple teams committing to the same repo you need controls over who has permission to commit to which directories and maybe a policy for handling merge conflicts across teams. I'm not sure what the tooling is like around that, but I could see the benefits as long as someone very high up was on board and had enough of a technical mind to keep order.

As far as two repos becoming one or one repo becoming two, you can split and merge repos while keeping the commit history.

edit: Do you have experience working somewhere with a monorepo that stretched across multiple teams? if so, what was it like?

I think it really just came down to dividing the repos into chunks that the deployment scripts could use/trigger off of. Instead of developing in a way that makes sense for developers and bending the deployment to fit. Since it was all in one directory on his computer, it was basically a mono repo from his perspective. Committing from the IDE just committed to whichever repo was changed, and since he was the only developer he never saw the downside. When I had to take it over ghorg[0] really came in handy. It's a script to clone all repos from a user/organization on gitlab, github, and others. Then once I opened up all the repos as one pycharm project I was able to get some stuff done, but at that point I might as well just had one repo with a separate directory for each.

[0]https://github.com/gabrie30/ghorg

EDIT: I also just remembered that gitlab is much better than github if you are going to go the route of multiple repos for one project. Gitlab lets you create namespaces to group your repos, so if they're all in the same namespace, you could have documentation to just tell people to clone all of them.