> Any downstream library team can instantly tell if their change has broken any upstream users because a good CI system will just automatically run all tests of all relevant projects. That way a library developer can either iterate until all projects depending on it are green, or the library developer can even proactively change all dependant projects and roll all fixes out in a single atomic commit.

Any proper (recent) package manager will do the same with a good CI without having any of the mono-repo disadvantages ( duplicated libraries, diamond dependency mess ).

The only real advantage of mono repo is that editing simultaneously multiple software components with API breaking changes is made easier, much easier.