Those are "the systemd requirements for the filesystem", what systemd needs. It provides no clarity about what goes _in_ those directories, nor why, Compare this checklist with the actual FHS documents, and you'll see the distinction. The result is confusion, such as the systemd migration of "/media" attachable storage to the individual user's owned subdiroectories in "/run".
Allow me to restate your comment:
> Systemd will continue to work for Linux distros that don't care for stateless boot etc
Rather, systemd will continue to re-arrange, and break, stable subsystems without warning to the user communities. OTher examples abound. For example, the new replacement for /etc/resolv.conf says at http://www.tin.org/bin/man.cgi...
Note that /run/systemd/resolve/resolv.conf should not be used directly but only through a symlink from /etc/resolv.conf.
Who's going to maintain the symlink? If any sysadmin unsuspectingly edits that file directly and breaks the symlink, or exposes their system to puppet, cfengine, chef, or any other system tool that manages /etc/resolv.conf, does it break the link? And what restores the link if it's broken? If the link is broken, DHCP updates to /etc/resolv.conf will no longer be effectively published by the sytemd based DHCP client.
I'm not even going to mention what happens if you accidentally follow generations of sytem standard behavior and install NTP alongside an active systemd NTP daemon. It's not a pretty site, and the accumulated clock management confusion if they're not pointed to the same NTP servers can break Kerberos authentication in startlingly short order.