A setuid binary can also check who is logged in and at what terminal. Ultimately it would do it in exactly the same way as the service program connected to the other end of something like dbus / bonobo.Just for the example of shutdown, it's a pretty common requirement that you want to allow a local user to do so (if logged in and running a session), but not the same user when connected from remote.
You could just as well argue X11 is "trash". With screen locking, problems exist for sure:
Just had a quick flick through these issues. The first one is due to some naff KDE theme problem, not X11 so I will jump past that. The second one says that X11 screenlockers can be blocked by other programs connecting to X11. Luckily in "modern" X11 that is impossible because X11 doesn't listen on public or TCP sockets (you probably know this but so many others don't!). No program can communicate with it but trusted ones any more than in Wayland compositors.
Re-inventing the wheel (badly) and calling it dbus only to remove the X11 messaging system calling it cruft is absolutely incorrect quite frankly. Good luck to them. It is a train wreck that will be fun to watch from the sidelines.I think the basic idea isn't all that bad (remove a lot of cruft nobody uses any more, focus on the main job…)