If I correctly remember the post by Lennart that spawned this entire debate, there were and are genuine technical reasons why a separate /usr filesystem doesn't really work anymore. Perhaps fixable _if_ all package developers (other than init) paid attention but that's not going to happen.
Now of course there is some leap from the above to making /bin and /usr/bin the same _directory_. AFAIK there is no good reason for that other than making it easier to write initscripts (or units). I'm not going to opine how good a reason is that :P Myself, I just have /usr on the rootfs. I don't have an initramfs; when I need a "rescue" environment I boot from a USB stick, not necessarily gentoo. Devuan seems to be the best for this kind of thing nowadays. -- Please don't Cc: me privately on mailing lists and Usenet, if you also post the followup to the list or newsgroup. To reply privately _only_ on Usenet and on broken lists which rewrite From, fetch the TXT record for no-use.mooo.com.