No-one defended cgroups. What you said about a single global management process was just plain wrong. I do find it amusing that you erroneously think that other people are lecturing you, by the way. (-:

A control group on the machine in front of me tells me that you are wrong about two more things.

    jdebp %ll -a /sys/fs/cgroup/service-manager.slice/user-services@.service/user-services@jdebp.service
    total 0
    drwxr-xr-x 6 jdebp root  0 Jun 29 18:17 .
    drwxr-xr-x 3 root  root  0 Jun 29 18:17 ..
    -r--r--r-- 1 root  root  0 Jun 29 18:18 cgroup.controllers
    -r--r--r-- 1 root  root  0 Jun 29 18:18 cgroup.events
    -rw-r--r-- 1 jdebp root  0 Jun 29 18:17 cgroup.procs
    -rw-r--r-- 1 root  root  0 Jun 29 18:18 cgroup.subtree_control
    drwxr-xr-x 2 jdebp jdebp 0 Jun 29 18:17 me.slice
    drwxr-xr-x 2 jdebp jdebp 0 Jun 29 18:17 per-user-manager-log.slice
    drwxr-xr-x 3 jdebp jdebp 0 Jun 29 18:17 service-manager.slice
    drwxr-xr-x 2 jdebp jdebp 0 Jun 29 18:17 system-control.slice
    jdebp %                                                                              

Unprivileged subtree delegation exists, that being a control group delegated to my account which has a whole subtree of further control groups in it, managed by multiple unprivileged processes. Your problem with "rootless" containers is not because of the non-existence, because Tejun Heo "isn't interested", of something that visibly exists. That's clearly not a correct description of the situation at all. Furthermore, https://lkml.org/lkml/2017/6/25/4 and https://lkml.org/lkml/2017/6/25/6 tell me that far from "isn't interested", Tejun Heo is interested in subtree delegation to unprivileged users. After all, xe is fidding with it right now.

systemd is not the sole user of version 2 control groups.