I found this fascinating. I too believed /bin and /usr/bin were discrete because /bin was for system stuff and /usr/bin was for user stuff. This post indicates

I found this fascinating. I too believed /bin and /usr/bin were discrete because /bin was for system stuff and /usr/bin was for user stuff. This post indicates it was due to disk limitations, leading to /bin being placed on the same disk as /usr, thus /usr/bin.