>But if I go from major version to major version, suddenly selling my MBP gets complicated, because I can only install the version that it came with after I wipe it.
That isn't true. You can install a fresh copy of the most recent OS.
You can, but that requires you to log in with your Apple ID and password. Or to download (with your Apple ID and password) a copy and create an installation thumb drive.
That isn't true. You can install a fresh copy of the most recent OS.