Comment by JdeBP

The problems with what you say are that:

1. The history of /usr subdirectories is a lot more complex than that. There was a /usr/lbin once, for example.

1. /usr/local is not where third party softwares from packages/ports go on "the BSDs". On NetBSD, they go in /usr/pkg instead, again exemplifying that this is quite complex through history and across operating systems.