The expected scenario, and the purpose of os.fspath(), is to enable high-level libraries to not know or care if they receive a pathlib object or a string.
In other words, they already have os.path.join() and os.path.split() calls, and currently break noisily if a pathlib.Path is passed in; by enhancing os.path to accept a pathlib.Path object that high-level library can start working with pathlib.Path objects without changing a thing, which means the user of that library can use pathlib.Path painlessly.
Unless I have seriously misunderstood something, os.path will be changed to work with __fspath__ objects.