Nobody is replacing the filesystem interface. The problem is just that because it IS a filesystem interface, people added knobs and exported information like there is no tomorrow. We would never have been this reckless with system calls even though both are unchangeable ABI.
While the rework of the cgroups interface is coordinated with systemd people, cgroups won't be based on or depend on systemd.
Cgroups are going to be more consistent across controllers and they are going to assume a single agent setting up and administrating a group (could be systemd, could be your custom management script, could be you from the shell). There is a ton of cruft in the code that bends over backwards to support multiple users accessing the control and info knobs _at the same time_ but nobody is doing that anyway.
You can still delegate if you like, but delegate to a single entity.