Have you used SteamOS? I don't know the particulars but I'm pretty sure they do indeed make all the important bits read-only. You can temporarily use pacman as root, but it isn't supported so any changes made are overwritten when an upgrade happens or a system crash causes a the partition to be verified.
Again, very much out of my depth here so sorry if i'm misusing terms etc.
Again, very much out of my depth here so sorry if i'm misusing terms etc.