I think the only thing that I find kinda strange about setuid/setgid is the fact that it is tied to an executable rather than as part of the executing principal.
As an example of an OS that doesn't use a concept, Windows only recently got Unix domain sockets (which is kinda the standard for IPC in *nix land) and generally used named pipes, mailslots, etc for IPC, which can be ACLed. Communication with services and elevation after Windows XP[1] was based on the the user's privileges and not "uid == 0" or "bit set on a file"
[1]: Before Vista, a lot of services actually straight up did show UIs on the desktop or whatnot. It was found though that doing this was pretty bad as you could use automation tools to drive the UI and it could lead to some pretty nasty local privilege escalations.
As an example of an OS that doesn't use a concept, Windows only recently got Unix domain sockets (which is kinda the standard for IPC in *nix land) and generally used named pipes, mailslots, etc for IPC, which can be ACLed. Communication with services and elevation after Windows XP[1] was based on the the user's privileges and not "uid == 0" or "bit set on a file"
[1]: Before Vista, a lot of services actually straight up did show UIs on the desktop or whatnot. It was found though that doing this was pretty bad as you could use automation tools to drive the UI and it could lead to some pretty nasty local privilege escalations.