Nice backronym, but /usr/ken (etc.) in v6 shows that /usr was originally a user disk.
You could ask Ken Thompson (/usr/ken) himself to verify. ;-)
Given that he is THE Ken Thompson, I'd argue that he was, in fact, the UNIX system resource.
Bahahaha. Same probably goes for everyone else in /usr/ like /usr/dmr.
That's how I've learned it around 1995, and that the division between /[s]bin and /usr/[s]bin has the reason that the former are required for system boot, while the latter are mounted via NFS from a central server - why waste local disk space when these would be identical for every system anyway (in a homogenous environment, at least).
While that made a lot of sense to me back then, I've become aware that it might not be absolutely accurate, though.
NFS is a Sun thing. If you consider Sun as the canonical historical Unix, this is still correct.
Sun had a few interesting early views on networking.