I see this whole discussion breaking down into a few groups which changes what gets done upfront and what might be done farther down the line:

1. Maximum acceptance: do whatever we can to make all representation of paths just work, which means making all places working with a path in the stdlib accept path objects, str, and bytes.
2. Safely use path objects: __fspath__() is there to signal an object is a file system path and to get back a lower-level representation so people stop calling str() on everything, providing some interface signaling that someone doesn't misuse an object as a path and only changing path consumptions APIs -- e.g. open() -- and not path manipulation APIs -- e.g. os.path -- in the stdlib.
3. It ain't worth it: those that would rather just skip all of this and drop pathlib from the stdlib.