I guess I just don't see how this is any different from using external libraries. Yes, bumping a library from v3.2.4 to v3.2.5 can sometimes break your build. So, the solution is we stop using versioning?

No, you just pin your app to v3.2.4 until a working update is made (either to the library or to your own app). I'm not sure why that suddenly becomes an impossible task once the creators of the app and the library work for the same parent company.

> Knowing when no one uses older versions is also hard

That's the thing though - you don't have to care. If you delete an interface, just bump the version number accordingly. People who still need the interface will stay on the old version. (And - bonus - they are now placed under implicit pressure to refactor their code away from using that old interface, now that they know it's deprecated and will no longer be maintained. By contrast, if you have a practice of keeping code around code just because people are still using it, that code tends to just stick around forever.)