Full disclosure: I used to work on this functionality at Microsoft.
Anyhow, sandboxing is just as necessary on a single-user OS as on a multi-user OS. And it's much more complex than multi-user support.
Never mind that the first threat scenario contemplated is a physical attacker is way out there on the probability scale unless you already suspect you are on someone's hit list.
I don't see what that has to do with multi-user systems though. If your argument is that we could have the Secure Boot system ask for the passphrase and tie the entire box to a single user... then you're missing out on most of the current point of multi-user systems.
The first is that many companies actually do have multiple people using the same machines. Not at the same time, but at different times. This needs auditing - i.e. a multi-user system.
The second is, again, auditing - when a system administrator runs a command on a system remotely, they do it as their own user.
The third is security (combined with auditing) - various service processes get run in different user contexts so that they can't mess with the user's stuff unless they're allowed to, and they have their own user ID that anything they do happens under.
Operating systems aren't built for home users, they're built for companies, in almost all cases, and stripping out the multi-user framework would change the OS to be unrecognisable. Just stripping out the authentication part doesn't buy you much complexity reduction either.
(First though: Windows now supports full disk encryption and secure boot. It certainly did not when I and it parted ways back in the days of XP SP1 circa 2002.)
I was not implying that the secure pass phrase/secure boot/etc be considered the basis for a secure mobile OS. Much the contrary. Multi-user systems with privilege hierarchies are fundamental aspects of how we now architect even our single-user devices. (Discussing whether another system is possible, desirable, and/or whether we could have or will eventually go down that route is midway between hypothetical and counter-factual.)
In any case, I believe Android uses the multi-user features of Linux as a security mechanism (and building a new kernel from scratch might not have led to Android being a major player - ARM companies already knew how to write device drivers for Linux), although it could reasonably use an object-capability system under a more focused kernel.