I don't know. Given what I know about how cgroups and systemd works, I don't think there's any particular issue with having a thin shim and starting systemd as PID 2, and so getting some of the reliability gains this article talks about. But I don't have all that much knowledge of the details and there may be good technical reasons why that is a bad idea (the man page notes that it is possible via a command line flag, but that it is not supported and only intended for testing). There are some issues surrounding cgroups moving to a single-userspace-arbitrator model, but I don't know what requires this to be PID 1.