At present multiple processes can set up and manage cgroups, but the maintainer wants to change to there being a single user space manager.

Thus systemd was pushed as "the" cgroups user space process.

You can already seeing the systemd devs behaving as if this was a done deal.

Ran into a email a while back about systemd clobbering a libvirt managed cgroup. And in it Poettering "suggested" that libvirt should hand control over to systemd as it would be THE cgroups manager going forward.

You can probably find similar encounters between, say, Docker and systemd.

It also looks a lot more like systemd in systems with good process supervision like Solaris.

Not at all. SMF supports delegated restarters, its milestones can contain far more state information (and explicitly) than targets can, it uses a configuration repository which enables runtime dynamic service state modification at a far higher rate than the relatively static systemd, its dependency system is simpler, it's integrated with the hardware fault manager (also to provide service resource identifiers beyond the COMM name), its profiles are more granular than presets, it has explicit service instance support and logging is flat file-based in /var/svc/log.

SMF has plenty of its own issues, but it's quite different from systemd. I think the delegation aspect is one of its most valuable lessons, but ultimately I also believe it's too overarching for a modern init.