I haven't tried in Git, but with Mercurial merging repos is as simple as pulling from an unrelated repository and merging, that's it. It's a lot simpler than splitting a repo up unless you accept that all of the old history can remain, then you just make a clone and delete what should no longer be a part of the repository.

But monorepo leads to tight coupling, and that is just as much a pain to work on as versioning, or two teams are simultaneously working on the same shared code, and you have not only merge conflicts, but conflicting functionality.