If you want to run post-upgrade, you got to do it yourself.
That’s just wrong. Pacman has hooks, and easy hooks are one of the reasons Pacman is loved. In a normal weekly upgrade I see Pacman run over 30 hooks. I do not think simply not updating user-modified config files is just the bare minimum needed.
I think this boils down to Arch’s philosophy: the users should know their system, and when something could break things, don’t assume things and do it automatically; have the user do it instead. Thus when shipping config updates to a user who had already changed their config, Arch does not overwrite the configs and instead ships the updated vanilla config with a .pacnew suffix. The user is expected to review such pacnews, a process that’s just like normal git merge conflicts when you use the pacdiff tool.
Good thing there are community frontends.