On Archlinux it is not recommended to update only one package with the package manager pacman. Let’s say I have 11 packages, and one of them is extra/firefox (true story). Updating only a pacman -S firefox could introduce problems, but installing a new single package if it wasn’t there is okay.
So my question is, could we get around this by removing and installing the same package again in one go: pacman -Rs firefox && pacman -S firefox
And where does it download the newly installed package from? It’s not in your cache, because you haven’t had it installed before and the remote server only has the newest version.
The download will simply fail if the version pacman wants to download isn’t available on the mirror. The version is part of the download URL.