Postponing the required Z change to later could be seen as beneficial in some scenarios but what if the change you made to lib A was a security fix, then you would want all apps of that lib to be forced updated right away. Then your your change should be backwards compatible, monorepo or not.
If you want to have reusable components then make sure they are reusable, if you want a special version of lib A that only works with lib B you are essentially forking lib A making it not longer a reusable lib, just a subdir for project B. Interface versioning could help with such non backwards compatible changes, in a monorepo you normally do this with a /2.0-directory.
