It's very much possible to make changes to internal libraries used all over the place, but it does require versioning to be something that people think about, a

It's very much possible to make changes to internal libraries used all over the place, but it does require versioning to be something that people think about, and a mechanism by which those libraries aren't just pulled from source control to depend on them. Once you've got some sort of dependency management such as an internal gem/npm/whatever source you can treat those internal dependencies the same as you'd treat external ones, instead of having to somehow coordinate a release of absolutely everything in one go.