For those wondering about the distinction between "system" containers and "app" containers, the difference is user namespacing -- I think we should stop using the naming difference but instead go with specifying that the containers "full user name spacing". User namespacing is the "magic sauce" for projects like Podman and a related project Buildah which builds images without root privileges -- the combination of user namespaces and FUSE.
BTW for those wondering if container orchestrators like Kubernetes will be shaken by this -- they very likely won't. Kubernetes has sidestepped this problem by introducing and relying on RuntimeClass[0], which allows you to swap out the runtime that's running underneath kubernetes. In fact, there is already a shim being worked on[1].
[EDIT] - While I'm here talking about container tech, please check out containerd[2]. It's been my runtime of choice for a very long time, has been very well maintained, and received many of the really advanced features first (ex. untrusted runtimes). IMO it should be the default container runtime of k8s.
[0]: https://kubernetes.io/docs/concepts/containers/runtime-class...