* having separate `/` and `/usr` filesystems has been useful in the past for booting without initramfs onto a minimal root filesystem that carried just enough to mount the `/usr` filesystem later in the boot process. Given the evolution of physical hosts' capabilities, initramfs'es have been default in Debian (and elsewhere) for a long time, and most systems no longer have an intermediate state during boot in which they have only `/`, but not `/usr`, mounted. Booting hosts through that intermediate state is not systematically tested in Debian anymore.
* another use-case is to share system files from `/usr` between hosts (over a network link) or containers (locally) which use different data or configuration. Having all software under `/usr` (instead of spread between `/` and `/usr`) makes the centralized update and the sharing easier.
-* the packaging infrastructure to install files outside of `/usr` is not standard and represents technical debt:
+* the packaging infrastructure to install files outside of `/usr` (e.g. installing libs under `/lib` instead of `/usr/lib`) is not standard and represents technical debt.
* given its status as remnant "folklore", the distinction between what _needs_ to be shipped in `/` and what can stay in `/usr` is often interpreted arbitrarily;
* allowing shipment of identically-named libraries or binaries in different paths can confuse common understanding of paths precedence.