This isn't a very good analogy.
> I think that RedHat IS at fault in this, because their design choice of designating systemd as the single writer was extremely anti-standardization.
This doesn't make sense. When the changes hits the kernel, all OS maintainers will need to choose a parent process for all cgroup control. Supporting this feature in systemd makes perfect sense and fits very well in the process hierarchy.
> Would people accept the fact that they would have to use KDE and interface with KDE in order to do containers on Linux?
Users and distributions who do not want to use systemd (their choice!) can use any other init system they want. They are also free to implement any other solution as their cgroup controller. Their situation is not changed one iota by this additional feature in systemd. They would have had to implement a cgroup controller either way.
In short, you are not required to use systemd if they make this choice. You may not have the benefit of someone else implementing the solution for you. But unless you're paying developers for the work, the guarantee of others doing work for you in the way you want them to is never provided.
> RedHat could have created a service "cgroupwriter" which would have communicated via D-bus or something.
They could have, but it would have added needless complexity. A significant number of tasks systemd needs to perform includes dependencies between cgroups. So mine as well manage it in the init system.
This isn't like a hard dependency on systemd. It just means that someone will need to author other solutions if systemd is not wanted.