Arch replaces _unmodified_ config files when changing. It’s not an uncommon behaviour in software to update defaults to the new defaults.

If you have a modified config file, it puts the new default one in a .pacnew file for you to compare, which seems strictly better to just deleting the new default one.

Huh you're right, I must've confused myself by removing/installing instead of upgrading recently.

Anyway I think the discussion boils down to semantics. ArchLinux is not "unstable" in the sense that it is prone to breaking. But it also delivers none of the stability promises that stable release distros or rolling release distros with snapshotting and testing like OpenSUSE Tumbleweed deliver. To call ArchLinux stable would make every distribution stable, and the word would lose all meaning.

Most distributions promise that an upgrade always results in a working system. Instead moving the manual maintenance to major release upgrades.