>While we're at it, why don't we add a real Plan-9-style bind and use union mounts?
There have been a few stabs at a union fs. The most recent one, OverlayFS was merged in 2014.
The thinking behind keeping the "weird historical" /usr is that it parallels other operating systems that (mostly) store their stuff in a subtree that is under a single subdirectory of the root. Think \OS2, \DOS, and \WINDOWS for starters. Also remember \LINUX from the world of LOADLIN.
Some programs may have strings like /usr/share hardcoded in them. I once tried renaming the root user to "boss" and it unearthed few issues of exactly this kind.
# ln -s / /usrNo, some people want /usr as a separate partition.
This hasn't had a genuinely useful purpose for many years.
On a modern package-managed Linux distribution, both / and /usr are under the control of the package manager, and separating them doesn't make sense since they are modified in lockstep, and used as a coherent whole.
And while in the Linux world we're busy "unifying" these locations, other systems can have the whole system on ZFS, where these locations can be in separate datasets if desired, and the whole lot can be snapshotted at will. Since it's all in a single pool, there's no need to even consider splitting it.
While we're at it, why don't we add a real Plan-9-style bind and use union mounts?