I think we should reconsider what a user is conceptually. Many, if not most computers, are only used by a single individual (particularly if we count smartphones); but that human user might want to establish a bunch of different contexts or permission modes for various software on their computer depending on what they want to do at any given time. IIRC Android achieves per-app isolation using its Linux kernel by giving every app its own user (in the Linux kernel sense of a user), which is of course pretty different from Dennis Ritchie logging into a PDP-11 with dmr in the 1970s and having that string be associated with his personally-meaningful data artifacts.