The claim is that the Arch pacman mechanism (and the dpkg one, and the rpm one, and ...) only prompts you when there is an actual merge, and when it does that,

The claim is that the Arch pacman mechanism (and the dpkg one, and the rpm one, and ...) only prompts you when there is an actual merge, and when it does that, it's actually fine and preferable for the package manager to leave old and new versions on disk and ask the sysadmin to resolve the merge by hand, and this works in practice. It does not ask you to review every change to /etc. This is why Arch is mentioned, as an example of a system where this situation works fine. (Debian and Fedora are also good examples.) If the BSDs' upgrade procedure cannot distinguish between changes to /etc where the local sysadmin has also modified the file and changes to /etc where the file is unmodified, and they prompt you to review every single change to /etc, that seems like an easily fixable missing feature. Do the BSDs really prompt you to accept every change to /etc on upgrade for files you didn't modify, such as sshd.conf or services?