I think the idea at the time was that if /usr is unavailable, you won’t be doing much with the system anyway (other than fixing the configuration).

Nevermind, apparently the original meaning had nothing to do with a network (TIL for me), so our discussion is kinda moot. See section 0.24 in this 2.9BSD (1983) installation guide

Locally written commands that aren’t distributed are kept in /usr/src/local and their binaries are kept in /usr/local. This allows /usr/bin, /usr/ucb, and /bin to correspond to the distribution tape (and to the manuals that people can buy). People wishing to use /usr/local commands are made aware that they aren’t in the base manual.