That's a nice excuse for /usr/bin but it is not true. /usr was intended to be user-owned directories, and all the commands a user was expected to type would reside in /bin (and .). Early AT&T Unix systems had multiple disks and made "/usr" be the "big" one, since they mistakenly thought that documents and things users worked on would be far larger than the system. Then what happened is the system disk filled up as more and more commands were added, and they needed space to put more commands, and since $PATH meant the "/bin" was not hard-coded in most places so it was easy to search other directories, and the lack of symbolic links or union mounts at the time meant the only way to put something on the /usr disk was to make it a directory under /usr, they added /usr/bin and started populating that. Eventually this also happened with libraries and /usr/lib was added.
Some people then started the excuse that /bin was for "system binaries" as opposed to "user binaries" but the distinction was pretty random. Not only that, /bin eventually filled up with "system binaries" and they had to add /usr/sbin!
Eventually so much stuff was put in /usr that people stopped putting home directories there. A different directory allowed them to be on a different disk than the system which was now about 90% under /usr/bin and /usr/lib. Almost everybody used $HOME rather than any hard-coded values so this was possible to change. First /users was the new directory but this was changed to /home more recently).