sudo is not installed by default on many distros (e.g. if you give a root password on Debian install, then sudo is not installed, but if you don't, it is so you can admin the system), so unless systemd introduces something to break sudo, I don't think it'll go away (it just may get bumped down the list of important packages). su seems to come from https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/, which is all the truly core stuff, and I don't see su disappearing from there.
