Further history of Unix of /usr

/usr/*, /bin, /sbin, /lib* are static under normal system operation: they only change when you add, remove or upgrade software (with a possible exception for /usr/local, but that can easily be a mount point or symlink if desired). That's one of the reasons for considering them to be "more similar than they are different", which makes unifying them into /usr make sense.

The rest of the directories in the root (in particular /etc, /home, /var, /srv) are normally mutable: they change under normal circumstances (either normal system operation, or sysadmin reconfiguration).

/etc needs to be on the rootfs because it contains the machine-specific configuration to know what additional filesystems need mounting (/etc/fstab, /etc/systemd/system/*.mount). It should be separated from the static part of the system (/usr) because it's inherently machine-specific, and because changes in /etc need to be preserved across upgrades.

In embedded scenarios, it's very appealing to be able to carry out upgrades by running with /usr from partition A, unpacking a new /usr into partition B, rebooting with /usr from partition B, rebooting into A to roll back the upgrade if B fails, and exchanging the roles of the two partitions for the next upgrade. OSTree is similar, although it uses a variable number of btrfs subvolumes instead of a fixed number of partitions.