Without a mono repo and the ability to build from HEAD all the components, it's much harder to be sure a change to a library _is_ actually backwards compatible

Without a mono repo and the ability to build from HEAD all the components, it's much harder to be sure a change to a library _is_ actually backwards compatible (think a complicated refactoring). Otherwise, there is much more fear that a change to an important library will have downstream impact and when the impact does arise, you've moved on from the change that caused it.